GSD_crosshair
Software that keeps a crosshair (aiming mark) of any image you like displayed at the center of the game screen. It works with software that uses DirectX (32-bit). It is built on GSD, a DirectX hooking library.
Download
Windows 7 (32/64-bit): GSD_crosshair(20110701).zip (PGP)
It appears unable to draw over DirectX 10/11 or 64-bit software. I have not tested other environments, but it might work.
Customization
Since crosshair.png is what gets drawn at the center, you can replace it with any image you like. By setting the alpha channel appropriately you can also make specific regions transparent. The bundled default crosshair.png is a size-adjusted version of one made by Poohaltea.
Doesn’t this count as cheating?
For how GSD works and how it relates to VAC (cheat detection), see the report on how GSD works.