Download ATI Tray Tools, which includes the useful shaders (HDRish and HDRish-lite) and lets you enable them through its game profiles.
You then need to get
this dll package. Extract that somewhere and rename the two 6.3 files to atioglxx.dll and atioglx1.dll. You can delete the other files.
For FS2, you can just drop the two dll files into the game directory and it will use them automatically. In general though, this will not work. You instead need to replace the equivalent files in your windows\system32 folder. You will want to keep your existing OpenGL DLLs though, so move them to some other folder first and then put the two 6.3 files in their place. I made a small batch file that lets me quickly switch between the two sets of drivers, so I can use the 6.3 files in the games where I want the effect and the 7.5 files for other games.
This feature only works with OpenGL games, by the way. I mainly use it (the Lite version) with several Unreal-based games, such as Deus Ex, and a few others like Descent 2 and 3.