Just to start things off...

http://mudomario2.blog134.fc2.co
A lot of sheet music; the things this person plays are amazing!!! (Must need basic understanding of Japanese to navigate)

http://maouscore.web.fc2.com/
Fairly hard sheet music, but they sound very good when played well. >.< And everything comes with a MIDI file!

http://josh.agarrado.net/music/anime/index.php
Basically a mass collection of all kinds of anime songs, as well as a pretty good selection of video game music... There are also links to other sites (Ichigo's, Sperion no Sora, etc) ^.^

http://www.animekeys.com/sheetmusic.htm
Not the best site for PIANO sheet music, but you can get Violin, Viola, group, etc music as well here, if that's what you're looking for... >.<"""

http://symphony.atspace.com/index.html
Use the navigation panels on the right for the master lists of anime, video game, and 'other' sheet music. >.< Pretty good selection.

http://www.youtube.com/user/marasy8
This person plays BEAUTIFULLY... Unfortunately, there aren't any sheets for what he does, except for when other people make them, in which case you'd need to search for '(name of song) marasy version' or '(name of song) as played by marasy', etc...

http://www.freewebs.com/zetaremix/sheetmusic.htm
Pretty much the same a Josh's site, but it looks different... >.<"""

http://sebastianwolff.info/
This guy is a musical genius -- and he does sheets for How to Train Your Dragon!!! *o* Yeah, I love that movie. It's beastly. Pun intended. ^.^ He also does other American sheet music, like the Other Father Song from Coraline, and stuff like that. >.<

http://vocalochu.blogspot.com/2009/12/vsq-list.html
Good Vocaloid VSQ's, UTAU UST's, MIDI's, and sheet music... I guess it's more helpful if you have the Vocaloid or UTAU program to convert the VSQ/UST to a MIDI file... >.<"""

http://animenz.wordpress.com/list-of-transcriptions/
This person is really good, too! An added bonus - they tell you the level of each sheet, so you know what difficulty you're downloading. ^.^

... That's all... If I find anything else, I'll likely put it up here. >.< Enjoy!