Try this website:

http://www.liveatc.net

you can listen in to ATC from a lot of different airports around the US
and the rest of the world.

Watch out for some of the B and C class feeds however, since you will
be getting the feed from a scanner that is constantly looking for
chatter on up to 10 different frequencies.
For the smaller fields, however, you can get a pretty good idea.

Chris