What did Sarah mean when she declared that whoever heard would laugh for her?
Rashi #1 and Targum Onkelos: She meant that they would be happy for her.
Rashi #2: She was hinting at the abundance of laughter that resulted from the fact that, on that day, many barren women were blessed with a child, many sick people were cured and many prayers were answered.
Targum Yonasan and Rashbam: Whoever hears about the wondrous miracle of the birth of her son will be filled with a joy borne of amazement.
Seforno: Everyone would laugh for joy for her and forget their concern for the baby's pain from the Milah, because, in spite of the pain of the baby, Hashem had filled her heart with joy.