Ichigo's Sheet Music is a website dedicated to anime and video-game sheet music. It was originally created by Spoonpuppet (forum alias), but is now run by Gand (forum alias). The forums is a great place to be which you can request in as you wait for transcribers who contribute to the site, to accept your request or direct you to another site which has the sheet you are looking for.
Josh's Anime Sheet Music Collection is a website run by Josh Agarrado, which currently hosts hundreds of sheets for nearly all instruments. Josh also performs on youtube so look out for him!
This is where I all started; helping out bishieXotaku transcribe a few sheets because I thought the list was a bit long. That's when bishieXotaku started to disappear and I started my own website and started full time transcribing. She has a small collection of violin sheets that is worth playing and pretty easy to learn.
This site is run by our very own Yarshe, our VG transcriber. It has many arrangements and transcriptions for many different instruments, be sure to check them out!