Sudoku Susser is another must have (solver, helper, much, much more):

Works on Mac, Windows, Linux

http://www.madoverlord.com/projects/sudoku.tSadMan Sudoku is shareware, but still great (solver, helper, generator):

http://www.simes.clara.co.uk/programs/sudoku.htmOnline solvers with no download required:

Paul's Pages (solver, helper, generator, archive):

http://www.paulspages.co.uk/sudoku/Easy to import and export puzzles without having to type them in. Can be used to solve manually, pencil marks, etc.

A SuDoku Solver (solver):

http://act365.com/sudoku/Scanraid (solver, helper):

http://www.scanraid.com/sudoku.htmSudoku Solver By Logic (solver, helper):

http://www.sudokusolver.co.uk/A fast solver on a Japanese page (solver):

http://www.inv.co.jp/~tanaka/java/numberplace/index.htmlThis one solves 3x3, 4x4 and 5x5 -- with or without DIAGONALS as well:

http://www.sf.airnet.ne.jp/~ts/java/numberplace.html