SPCs are the popular file format for music from Super Nintendo Entertainment System games. They are often ripped from roms with ZSNES. They are of identical quality to the SNES game, and of very small size. A typical music track is 64.5 kilobytes in size.
To play an SPC track, you can get the SNESAmp plugin for Winamp, or there are various SPC specific players available for download. The largest archive of SPC game soundtracks is available at http://www.zophar.net/zsnes/spc/. They usually come compressed in RAR format, because they compress much better in RAR then in ZIP.