I'll shout about what we use, since I very much doubt that anyone else here is using it (I think it was developed by a guy on our realm):-
http://www.wow-raidar.com/
Basically you sign up, create your toons, complete with resistances & ctprofiles if required, then someone creates your guild and sticks all the characters he recognises in it.
From then on whenever you log onto it, you can see all PuG and Guild instance runs that are scheduled.
Whoever creates a raid is leader for that raid, he can appoint deputies, who can then change people's status from "listed" to "drafted" and so on.
It might not be perfect, but it works a lot better than a lot of stuff I've tried.