Ultimate Soundtr/a/ck is something I discovered only a few months ago even though the whole project started around mid 2008. This 'Ultimate Soundtr/a/ck' is a large group of soundtracks with anime songs such as openings, fight battles, endings etc. As far as I am aware, this is the most comprehensive and best collection of such songs. This is because they are selected songs that are good or memorable, much better than getting an OST filled with songs you don't remember and instrumentals you don't care for.
This project is organised into the following sections.
These sections each have relevant music in them, such as TEARS being sad memories and such while HHHNNNG being cute songs. However while it is sad to say some of these sections/volumes are incomplete, their section on the wiki does provide Youtube links to the songs so you can download them yourself anyway.
Check out the main website at http://bluwiki.com/go/Ultimate_soundtr/a/ck and check out these albums.
You can go ahead and download any of the TEARS or HOTBLOOD volumes from here: http://www.mediafire.com/ultimate-soundtrack
The other albums are not available for download as complete albums because they are still being worked on.