Hi Karen,
The big affiliate networks are Commission Junction (CJ) (
http://www.cj.com)
and LinkShare (url=http://www.linkshare.com). CJ also owns BeFree. These two handle the bulk of the affiliate programs out there.
There are also a few other affiliate networks like ShareaSale and Performics but I haven't tried them out completely.
Then there's ClickBank (
http://www.clickbank.com) for digital products like ebooks.
You can also find independent affiliate programs by doing a search on Google or Yahoo for 'keyword' + 'affiliate program'.
You can join all of them, but no one has time to fully dedicate themselves to everything... unless you have an army of people working for you.
Good luck.
Hock