Here is what it looks like after typing in a puzzle (Linux version shown):
Then, after clicking Solve, it becomes
The GUI is pretty spartan; I haven't added the "tic-tac-toe" lines typically seen in the puzzle, nor do I distinguish the initial numbers from the ones the solver filled in. I may get around to those features, but probably not unless someone has a burning desire for them. I mainly wrote this out of interest in the solution mechanism itself, not the graphics.
Legal stuff: This program is free for personal use, and may be distributed freely. Modified versions, including rebranding or use of all or a portion of the code in another program, may not be distributed. This program comes with no warranty, express or implied. If commercial use or rebranding are desired, please contact me.
Download the program from here:
This program is a Java program; you need a Java installation to run it. Java is very easy to install, and is available here. Once Java is installed, you should be able to just double-click the file to run it.