Question: Does anyone know tools like [SpyStudio](http://www.nektra.com/products/spystudio/) to hook processes?
The features I would like to have are:

* log function calls on other processes
* GUI
* scriptable
* opensource

I already looked at some tools like:

* [SpyStudio](http://www.nektra.com/products/spystudio/)
* [Detours](http://research.microsoft.com/sn/detours/)
* [DynInst](http://www.dyninst.org/)

but none really fully satisfies me.

Answer: 
[WinAPIOverride32](http://jacquelin.potier.free.fr/winapioverride32/)

The primary usage of WinAPIOverride32 is monitoring/logging function calls of other process.
It also has plug-in framework, so you could write your own hook handler. And it is open source (GPL).
You can look at Detours library from Microsoft Research. Its a generic API Hooking library.