Gemara

(Mishnah - Aba Sha'ul): One who is Hogeh Es ha'Shem b'Osiyosav (pronounces the letters of Hash-m's name) has no share in the world to come.

101b (Beraisa): This is outside the Mikdash, b'Lashon Agah.

Avodah Zarah 18a: It was decreed that R. Chanina ben Tradyon be burned, for he was Hogeh Es ha'Shem.

Question: One who does so has no share in the world to come!

Answer: He did so for the sake of learning. (A Beraisa forbids learning witchcraft to practice it, but one may learn it to understand its laws and to rule about it. The same applies to being Hogeh Es ha'Shem.) He was he punished because he did so in public.

Pesachim 50a: Rava planned to publicly expound Hash-m's name. An elder remarked "(Zeh Shemi) l'Olam" is written (without a Vov, so that we can read it) Le'alem (to conceal).

Question (R. Avina): It says "Zeh Shemi l'Olam", but the verse ends "v'Zeh Zichri l'Dor Dor"!

Answer: Hash-m says 'My name is not pronounced like it is written. It is written Yud-Kei, but it is pronounced Aleph-Dalet'.

Shevuos 35a (Mishnah): If he made the witnesses swear (that they don't know testimony) by 'Aleph-Dalet' or 'Yud-Kei', they are liable (if they knew).

35b (Beraisa): Once one writes 'Aleph-Lamed' from Elokim, or 'Yud-Kei' of Hash-m's name, he may not erase it. One may erase 'Shin-Dalet', 'Aleph-Dalet', or 'Tzadi-Beis' from Shakai, Adon-i, or Tzevakos;

Sukah 5a (Beraisa): Yud-Kei was written on the top line of the Tzitz, and Kodesh Lamed (the first letter of la'Shem) on the bottom.

Eruvin 18b (R. Yirmeyah ben Elazar): After the Churban, it suffices for the world to use two letters (of Hash-m's name) - "Kol ha'Neshamah Tehalel Kah Halelukah."

Rishonim

Perush ha'Mishnayos (Sanhedrin 10:1, shortly before ha'Yesod ha'Rishon): Hogeh Es ha'Shem is Yud Kei Vov Kei, Shem ha'Mefurash.

Yad ha'Melech (Hilchos Teshuvah 3:6 and Avodah Zarah 11:12): Avodah Zarah 18 connotes that the Halachah follows Aba Sha'ul. Why didn't the Rambam bring this? Perhaps he did not include Hogeh Es ha'Shem with other Aveiros for which one has no share in the world to come, for one is eternally punished for the others. However, he should have taught Hogeh Es ha'Shem separately!

Rashi (101b DH uv'Lashon): Lashon Agah is a language of Nochrim. Or, Agah is like Ugah, i.e. amidst secular conversation. One may not explain the 42-letter name in Lashon Agah outside the Mikdash. If he does, he has no share in the world to come. In Lashon ha'Kodesh, he is not punished so much. In the Mikdash, since the custom is to mention its Perush, he is not punished.

Tosfos (Sukah 5a DH Yud Kei): Even though Yud-Kei is a proper name of Hash-m, one who says this does not transgress Hogeh Es ha'Shem, for be does not say the full name. Also, Rashi explains that the Isur is to expound the letters of His 42-letter name. Even so, people are careful about all the names.

Tosfos (Shevuos 35a DH b'Aleph): Even though one who is Hogeh the letters of Hash-m's name has no share in the world to come, perhaps it is permitted because he does not intend for the name.

Rashash: Perhaps Tosfos means like Tosfos says in Sukah (he does not transgress because he does not say the full name).

Sefas Emes (Sukah 5a DH b'Tosfos): Why is it more lenient if he says only half the name? The Isur is not only with intent! Tosfos in Avodah Zarah did not distinguish like this. The Gemara said only the first two letters for brevity. The Isur of Hogeh Es ha'Shem does not apply to writing it.

Har Tzvi (1:5): According to Rashi's second Perush, one may not be Hogeh the four-letter name, but the great Onesh is only for the 42-letter name. Tosfos and Perush ha'Mishnayos are like Rashi's first Perush. I am unsure if Hogeh Es ha'Shem b'Osiyosav is pronouncing the name as it is written, or saying the names of the letters. If it is the latter, perhaps this is only b'Miluy Osiyos (he says the letters that spell out each letter, e.g. for 'Yud' he says 'Yud Vov Dalet'). Tosfos (Sukah 5a) holds that it is even without Miluy. (Even if one says just 'Yud', we hear also the hidden part, i.e. Vov and Dalet.) Sefer Nagid u'Mitzvah cites the Ari Zal that Hogeh Es ha'Shem is b'Miluy Osiyos. Surely, the primary Perush is without Miluy, but one must be careful also with Miluy. Why did Tosfos ask about being Hogeh the two-letter name? Just like one may say the name itself, one may say the letters! The Isur to say Hash-m's name like it is written is only for the four-letter name! We must say that even though the Isur to expound is only for the four-letter name (like Rashi says in Pesachim), the Isur to say the letters applies to all names. The Maharsha in Avodah Zarah says that the Isur is to say the letters, but surely one may pronounce the word, for any name other than the four-letter name. We say in Eruvin 'it is enough for this world to use two letters of His name. He equates all names regarding the Isur to say the letters, for the Torah says "conceal".

Har Tzvi (1:5): Even if the Isur does not depend on intent, perhaps it is permitted if he does not intend for a Kodesh name. If one says with the MIluy, it is a Shem only if he intends for this. Tosfos did not ask about writing, rather, about the Beraisa, which people said in the Beis Medrash. The Radvaz (5, Leshonos ha'Rambam 1408 - I could not find this) says 'Hogeh Es ha'Shem b'Osiyosav is to say it without a Kinuy name of Adnus in every vowel. Tosfos explains that it is to say the letters. The words Hogeh Es ha'Shem b'Osiyosav connote like this.' He forbids both ways for the four-letter name, but not for Shem Adnus (one may say the letters). The Yerushalmi says that Kusim who swear illustrate Hogeh Es ha'Shem. Why does it teach about Kusim? I answered that it teaches that it is forbidden even without intent. I retracted. This is obvious! Rather, the Yerushalmi teaches that Hogeh Es ha'Shem applies even to saying the letters without MIluy. Surely a Kusi does not say the Miluy!

Tosfos (Avodah Zarah 18a DH Hogeh): Most explain that Hogeh Es ha'Shem is to explicitly read the letters of Shem ha'Meyuchad like the letters of other words. Pesachim 50a forbids this. The Ri forbids pronouncing Yud-Kei (i.e. saying 'Hei') like people often do, for this is Shem ha'Meyuchad. R. Elchanan forbids saying Aleph-Dalet from Adon-i, for one may not erase this, and not 'Yud-Kei' of Hash-m's name. This is like R. Chananel's Perush in Shevuos. However, the Ri permits (erasing Aleph-Dalet), like our texts of the Gemara. What is written in Seforim (in Pesachim, that we say Aleph-Dalet), is not because one may not say the entire name. Rather, it is for brevity. (This is how the Maharsha explains.)

Ramah (101b d Piska): Some say that Lashon Agah is (saying Hash-m's Inyan and Midos in) another language. I say that he changes the tradition for the vowels. Some say that it is amidst secular talk, or in the way of mockery. This is wrong. If so, it should have said 'Ugah' or 'Derech Agah', and the Onesh should be even in the Mikdash. If Agah in another language, there is no Onesh for this in the Mikdash, since there there is a Heter for Lashon ha'Kodesh.

Poskim

Rema (YD 276:13): L'Chatchilah one may not write Hash-m's name not in a Sefer, lest it come to disgrace.

Chavos Ya'ir 16: Hogeh Es ha'Shem does not apply to writing. We write Hash-m's name in a Sefer Torah, Tefilin and Mezuzah! It is disgraceful to write His name needlessy, lest it come to disgrace, and one must bury it, but it is not intrinsically forbidden to write it.

Chayei Adam (5:27): Even if one says 'Yud (immediately followed by) Hei' (e.g. in 'l'Shem Yichud...'), he pronounces Hash-m's name, and has no share in the world to come. Rather, he says Yud Kei.

Yabi'a Omer (8:11:7): Some say that in Korbanos, one should pause in between 'Abaye' and 'Havah', lest it sound like he says Hash-m's name as it is written.

Teshuvas R. Akiva Eiger (1:31): Writing is not like speaking, for we may write Hash-m's name, but not pronounce it. However, one could write it with intent that it not be like saying it! A mute may not tithe, for he cannot bless. If he would write the Berachah to be Yotzei it, he would need to intend that writing is like saying, and one may not say Hash-m's name. However, he could write the name like we pronounce it! Further, he could write the Berachah in another language (one is Yotzei in any language - OC 206), and then Hogeh Es ha'Shem does not apply, like it says in Sanhedrin. (Note - the Gemara says that one is liable only in Lashon Agah! - PF.)

Chasam Sofer 192: R. Akiva Eiger asked about Tosfos (Hagahos in R. Akiva Eiger Sanhedrin 90a, p.224 - for Hogeh Es ha'Shem is saying the word, not saying the names of the letters). This bothered also me. It seems that since the Tana taught that Yud-Kei was on top, and he did not finish the name, this shows that he forbids even saying all the letters. Tosfos asked, if so he should forbid saying even Yud Kei! People are careful about this, and say Yud Kei Vov Kei.