Software by kakuropuzzle.com:

Kakuro Cross Sums is the puzzle world’s best-kept secret. Called Cross Sums in the USA, Kakuro in the UK, and Kakro in Japan, it’s a fun, entertaining and highly addictive number puzzle game that will challenge and perplex you. The rules are simple, but completing a puzzle takes practice and concentration. Play unlimited Kakuro Cross Sums on your PC. Try it and see why millions prefer it to Sudoku!