There are lots of sites on the Internet. Unfortunately I cannot keep up with all of them. Here I present a few but IMHO important ones:

The ATM Page (A "must visit" site)

ATM FAQ (Frequently Asked Questions about ATMing, check here on how to join the ATM Mailing List)

Bartels, Mel (The best start point for links concerning all things ATM)

UK ATM Resource List, by Stephen Tonkin

European Resource List, by Wolfgang Birkfellner