Chess problems are a fun way to your keep brain agile.
Chess Puzzler contains more than 4000 STATIC diagrams of chess problems and
can be extended with your own problem sets.
Chess Puzzler is NOT a chess playing program and focuses entirely on STATIC diagrams of chess problems, in particular you CANNOT move the pieces - this is intentional, forcing your brain to work harder.
The app is small and can easily handle even large problem
sets of over 100000 problems, e.g. Eduardo Sadier's
excellent collection of "mate in 2s".
Please check the web link for more information about adding problems.
Support for fairy pieces is in preparation.