Tsumego, a Go problems mailing list - A free Go problems mailing-list. Problems set up by Fan Hui, 2 dan professional chinese player. - http://tsumego.jeudego.org/index_eng.php
Go test - Go test with 20 problems, compiled by Alexander Dinerchtein - http://play.baduk.org/
Go Problems - solve go problems interactively through a Java applet; explore variations and contribute problems to the community - http://www.goproblems.com
Minoru Harada's Tsumego - Elementary and intermediate weekly problems with archive and solutions. - http://www.hitachi.co.jp/Sp/tsumego/igohpe.htm
Tsume Go Of The Day - Easier and harder daily problems with solutions. - http://www.qmw.ac.uk/~ugah006/tsumego/