50btc.com - largest and one of the first mining pools in Bitcoin history
Bitminter.com - has their own custom mining application and merged mining with NMC (Namecoin)

P2Pool, P2Pmining.com
To learn more about P2Pool, go to
https://bitcointalk.org/index.php?topic=18313.0

Ozco.in - 1100GHashes, great pool, lasted since 2011