12 difficult problems  from our 1977 Calendar set up by Nick Wedd (note these are ASCII only).
A collection of over 1,000 problems, including the famous Korean Problem Academy and Ancient Wei-qi Patterns.
Approaching 20000 problems. A good variety of problems, in an interactive format. Anyone can add new problems to the site.
Two new problems each week, one elementary and one intermediate, by top Japanese amateur Minoru Harada; answers the following week. Around 2000 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.