A collection of over 1,000 problems, including the famous Korean Problem Academy and Ancient Wei-qi Patterns.
Over 9,000 problems. A good variety of problems, in an interactive format. You can see the solution, and have the latter shown in a choice of several formats. Anyone can add new problems to the site.
Two new problems each week by top Japanese amateur Minoru Harada; answers the following week. Over 500 problems so far. The site has separate Japanese and English language sections.
Each of these pages leads to 20 life and death problems.
Various problems and sets of problems. In particular, see:
A problem for every month from January 1996 to December 2005.