Incomplete list, let me know if I missed anything good. (I don't get paid from any of the links)
General information
Wikipedia page on 24 the math game
The "Official" 24 Game website
Online game, solver and source code
Solver1, Solver2, Solver3, Solver4
24 game in different programming languages on Rosetta Code
Solver in different programming languages on Rosetta Code
Online game 1, Online game 2 with solver
Apps
I have played and highly recommend Combine 4! on IPhone. Here are several other apps that might be interesting app1, app2, app3.
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