I was wondering why there are 108 Sekirei(s). I asked Ana-sempai, and she said it had something to do with a Buddhist belief; there are 108 sins or other.

Sekirei is not the only place where 108 pops up. For example, Yurie Hitotsubashi ranks 108 during her temporary transfer. I could swear I saw 108 elsewhere, quite recently even, but now I cannot remember.

Google suggests that Suikoden, a game for DS, has 108 playable characters. It's not an anime, however.

UPDATE: Omo reminds about Shikabane-hime, who, as I now recall, had to kill 108 corpses (don't ask, I have no idea what that means; something like Soul Eater with a bare ass, presumably).

David Mankins sent a link to a page with some of the things linked to the number 108, e.g. "There are said to be 108 earthly desires in mortals", etc.

DiGiKerot speculates that Sekirei may actually be hinted by Suikoden or at least its Chinese parent, 108 Stars of Destiny.

Steven Den Beste also found 108 in Negima, where someone gets "108 Carnal Boys".