Problems

997 Capture Problems
Free downloadable tesuji collection with verified solutions.
http://www.t-t.dk/madlab/problems

Find your go style
Answer 15 Go problems and get the estimation of your playing style. The test was compiled by Alexander Dinerchtein, multiple European Champion.
http://style.baduk.org

Go Problems
solve go problems interactively through a Java applet; explore variations and contribute problems to the community
http://www.goproblems.com

Go test
Go test with 20 problems, compiled by Alexander Dinerchtein
http://play.baduk.org/

Minoru Harada's Tsumego
Elementary and intermediate weekly problems with archive and solutions.
http://www.hitachi.co.jp/Sp/tsumego/igohpe.htm

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