Download using SVN:
http://corsix-th.googlecode.com/svn/trunk/
Download using HTTP:
http://th.corsix.org/corsix-th-r2697.tar.gz
Use http://th.corsix.org/corsix-th-latest.tar.gz
to get the latest version automatically.
Prebuilt win32 version:
SDL game-only: http://th.corsix.org/corsix-th-r2697-win32.zip (latest)
OpenGL game and MapEdit: http://th.corsix.org/corsix-th-r2697-win32-ogl.zip (latest)
Compatible with: r2038 - r2697 (if you're currently using version r2037 or earlier, then you'll need to re-compile the C++ code).