These are some of the sites that I found useful for anime music, or just ones that look interesting. I hold no responsibility for the content that they put up, and I don't know about the accuracy of their work either (don't have the time to run through everything, you know).
If you'd like your site to be added to this list, contact me and I'll consider it.
Anime Sheet Music
- Anime Scores
A great website that offers sheet music for selected anime songs (mostly those with piano in it). You'll need to be able to read music (those little swiggly things on the music score), but they have a forum so you should be able to pose any questions you have to them. If I'm not mistaken, they also offer transcribing services in the forum - you can request the staff to transcribe the sheet music for songs you want (unlike here, where I only do the chords), so if you're a bit more picky about accuracy then I recommend going there.
- Ichigo's Sheet Music
Another popular sheet music site, with some guides on transcribing and composing, and also a good selection of songs.
- josh's anime music sheet collection
One of the most active transcribers I've seen in a while. Josh runs a simple site, but he also maintains an anime sheet music master index, which indexes notations from as many popular sites that he can compile. He also takes requests from many forums, including here on AnimeChords (sheet music, not tabs!)
- Search-based Open Directory of Asian Sheet Music (S.O.D.A.)
This is an awesome searchable database of anime and japanese music sheet music. It lists content from AnimeScores, Ichigos, josh and loads of other popular sites. It will save you a lot of time scouring individual websites. Essentially, the Google of anime sheet music.
- Anime Scores
Anime Scores has guitar tabs too, although the selection is limited.
- アニソンを弾き語る (Japanese chords site)
A chords website entirely in Japanese. You'd have to be able to find the title of your song in Japanese script, but there are many songs already transcribed. A good resource for those who can use it. Thanks to xmetallicx for the link.
This site is incredible. It has a menu for you to select *any* guitar chord you want, and it will show you multiple variations of the chord in a graphic. An absolute MUST for guitar players.
Anime Music Karaoke
- Anime/J-pop Karaoke Box
As the name says, it's a Flash karaoke player that allows you to sing a small selection of tunes. Quite fun, but the songs run out pretty quick.
- Karaoke Machi
Also a Flash karaoke "machine" that features anime and J-pop songs. Page design is quite nice, and there's an okay selection of recent and older songs. The song quality is generally better than Anime/J-pop Karaoke.