ApiHooks 5 is a set of libraries and tools for advanced
process modification and noninvasive debugging
on Microsoft(R) Windows(R) 95,98,Me
and x86 NT,2000,XP,Server 2003,Loghorn and Vista x86.

Win32 API hooking library, utility

ApiHooks 6 is free for NON_COMMERCIAL use only.
Current Version : 5.72 - last version
Examples include Winsock, Registry, GDI, ExitWindowsEx hooking, cross-session communication via remote thread, etc...
Documentation
DefHook macros

Smallest, fastest, sophisticated hooking system
Everything is transparent, fully in the hands of developer
Supported languages: C, Delphi (COFF and OMF linkers)

Used by: Cisco Systems, Inc.; SoftStory,...