Life and death problems and related material.
Patrice Fontaine's site with go problems and various resources about go software on the Mac (Classic and OS X).
All kind of resources (tsume go, game records, ...)
Solve go problems interactively at this site, and improve your reading abilities. Right now (August 2006), more than 4500 problems are available, from beginner to strong player level.
Fan Hui 2p's life-and-death Go problems mailing list. Problems and solutions are sent once a week, as sgf files. You will receive 3 or 4 tsumego of different levels : 20 kyu, 10 kyu, 1 Dan, from time to time a 5 dan problem, and the solutions of the past week.
Lots of life and death problems (elementary and intermediate level).