What was Ya'akov referring to when he said "Dan Yadin Amo ... "?
Rashi: He was referring to Shimshon's final act of vengeance against the P'lishtim. 1
Ramban: Of the many times, during the early period of the Shoftim, that the P'lishtim subjugated Yisrael, Shimshon was the only one who defeated them, thereby avenging 2 Yisrael for the maltreatment at their hands. 3
Rashbam: He was referring to the entire tribe of Dan, who throughout the period of both Moshe and Yehoshua, 4 traveled at the back of the camp ('Me'asef l'Chol ha'Machanos'), and who were therefore required to defend Yisrael from whoever attacked them from the rear.
Targum Yonasan: 'From Dan there will arise a man who will judge his people in truth.'
Rashi: See also Shoftim (16:28-29) and refer to 49:18:1:1. We find the word "Din" with connotations of vengeance in Ha'azinu, Devarim 32:36. Refer to 49:16:1:2*. (See Rashbam's strong objection to Rashi's explanation that Ya'akov was referring specifically to Shimshon.)
The Torah uses the word "Yadin" because Shimshon was a Shofet and not a king (Ramban).
Ramban: As the Pasuk in Shoftim (13:5) indicates. This is also probably what Targum Onkelos means when he writes "in his days his people will be redeemed".
See Yehoshus (6:9).
What did Yakov mean when he added "k'Achad Shivtei Yisrael"?
Rashi #1: He meant that, at the same time, he also avenged all of Yisrael 1
Rashi #2: He was comparing Shimshon to David, from the special tribe Yehudah (who defeated his enemies - "Yadcha b'Oref Oyvecha" - Ramban). 2
Targum Yonasan: All the tribes of Yisrael together will submit themselves to him.
Hadar Zekenim: Do not say that because he is from a Shifchah, he will not have authority like his brothers.
מה הפרוש "כאחד שבטי ישראל"?
מהר"ל (חידושי אגדות ח"ב עמ' מ, ד"ה כאביהם, סוטה דף י.): מדובר על שמשון שדן את ישראל כאביהם שבשמים, כי הדין חייב להיות אחד ולא שנים ולשמשון היה את מידת המשפט והדין לגמרי. 1
מהר"ל (חידושי אגדות ח"ב עמ' מ, ד"ה כאביהם, סוטה דף י.): שמשון היה מיוחד בלי שיתוף של אחר- כמו השמש שאין כוכבים משותפים עמה, ולכן הוא הגן על ישראל.
עיין עוד במהר"ל שם (בעמ' לח ד"ה תחלת) שרמז גם על מה שנקרא דן נחש עלי דרך.