Also for Go:
playgo.to/iwtg/en/
www.online-go.com/
www.gokgs.com/
www.goproblems.com/
And sites where you can play several strategy games (chess, shogi, go, xianqi, etc):
www.littlegolem.net/jsp/main/
www.brainking.com/
www.ludoteka.com/juegos.html?hizk=en