All the projects mentioned in the Real World section and more. Join a project and experience distributed computing in action!

Casino-21: http://www.climate-dynamics.rl.ac.uk/
Computing Minimal Equal Sums Of Like Powers: http://euler.free.fr/
Evolution-at-home: http://www.evolutionary-research.org/
Entropia: http://www.entropia.com/
Gamma Flux: http://dcypher.net/
GIMPS: http://www.mersenne.org/
Internet Movie Project: http://www.imp.org
OGR: http://www.distributed.net/ogr/
Parabon: http://www.parabon.com/
PiHex: http://www.cecm.sfu.ca/projects/pihex/pihex.html
Popular Power: http://www.popularpower.com/
Porivo: http://www.porivo.com/
Prime Form: http://www.hut.fi/~nkuosa/primeform/
Process Tree: http://www.processtree.com/
RC5-64: http://www.distributed.net/rc5/
SETI@home: http://setiathome.ssl.berkeley.edu/

MSNBC's space site is a good place to go if you're interested in the SETI@home search. http://www.msnbc.com/news/SPACELIFE_Front.asp
For those interested in cryptography, RSA Security Labs has a very good FAQ: http://www.rsasecurity.com/rsalabs/faq/
A great resource for understanding mathematics and mathematic-related projects is the Prime Pages: http://www.utm.edu/research/primes/
Not sure which project to join? Read some reviews of different projects here: http://www.bottomquark.com/distributed.shtml