RYMix.org is a collection of Covers, Mashups and Remixes.

For browsing purposes;
A Cover is a recording of a song that was first recorded or made popular by somebody else.
A Mashup is a song or composition created by blending two or more songs, usually by overlaying the vocal track of one song seamlessly over the music track of another.
A Remix is a alternative version of a song, different from the original version.