I recommend LiveATC.net also, although try to stay away from US frequencies. RT is quite different in FAA land and to listen to it while still getting to grips with your own RT will only serve to confuse you. I recommend listening to the Irish airports (regional if possible i.e. EIKN which gets significant GA aircraft at weekends) as their RT is very very similar. Unfortuantely it is illegal to broadcast or listen to UK radio streams online (due to Ofcom I believe?).
Hope that helps,
Ryan