Austin Homebrew Supply.
I use it almost exclusively. Great flat shipping rate, great customer service, fast shipping, good selection, etc., etc. The guy who owns (I think? Maybe he just manages it?) AHS, who goes by the username "Forrest", is a regular around here too, and will often give special offers to HomebrewTalk members.
I'll use other places once in awhile, but only if the cost savings are phenomenal or AHS doesn't have what I'm looking for. Viable alternatives that I've had good luck with include Midwest Supplies and Northern Brewer.