Check out
@http://research.microsoft.com/sn/detours/. They've done a nice job with their injection library.
Mathias Rauen ("Madshi") also has a pretty sweet library for hooking. I believe it's the library WC3Banlist decided to use. You can find more info at
@http://help.madshi.net/madCodeHook.htm. This page gives you a nice run-down of some different ways you can hook a method:
@http://help.madshi.net/ApiHookingMethods.htm