Codebreaker comes from a line of similar analog games. If you like Codebreaker, you'll probably also like Mastermind and Bull and Cows.
This simple Web game was built as a demonstration of the exyus web engine. In geek-speak, exyus is a REST-compliant HTTP engine built using C# that runs on the Windows/IIS platform.
If you're interested, you can view the source code for this web version of the game. Most of it is written using XML/XSL and T-SQL. There is also a chunk of C# to glue it all together. The entire source code for the exyus web engine as well as the code for this game is available on the exyus @ googlecode site.