- Server-based correspondence chess:
http://www.queenalice.com/
lss.chess-server.net
che55.com
- Real-time chess:
http://www.freechess.org/ (also known as FICS)
And do not forget yahoo games ;)
- E-mail chess:
http://www.iecg.org/
http://www.chess-iecc.com/
http://www.cxv.com.br/ (for potuguese and spanish speakers)
