A cappella vocals are widely used in mash-up tracks. They are often extracted from CD tracks by an ingenious DIY method:
A CD single is located containing both a vocal and instrumental version of a song. Hopefully, the two tracks will be identical apart from the vocals. Both are ripped to uncompressed audio files - wav or AIFF. The rest is a combination of precision and mathematics, and no small amount of luck. The two files must be synchronised perfectly - this is achieved by visually editing the waveforms so that both begin at precisely the same point. The instrumental version is then inverted in a sound editor. By mixing the two files together, the instrumental can then be 'subtracted' from the vocal version. This should, hopefully, leave just the vocals - with a few unavoidable (and largely unnoticable) artefacts - ready for use.
Rap a cappellas are particularly valued, as their spoken nature makes them easy to integrate with another song. It's sometimes possible to find a CD single with an a cappella track included, which would be a huge time-saver. A cappella tracks are often traded via the web and file sharing programs. Some are more popular than others - Missy Elliot's Get Ur Freak On and Work It are somewhat ubiquitous.
A more detailed guide to this technique by a master of mash-ups can be found at http://4trak.net/dsico/archives/000082.html.