Besides Nashbar, Chrome and Performance, already mentioned, which have always been quick, cheap and friendly, my dealings with Timbuk2 and J & G Cyclewear have also been great. I tend to overanalyze things and ask lots of questions before pulling the trigger. Both Timbuk2 and J & G have answered my e-mails within 24 hours and shipping has been close to immediate.
My only bad/horrible experience was with a club in Iowa selling Guinness jerseys on-line. After about six months and multiple e-mails I just demanded my money back. (I was getting responses like, "...we're a club so the jersey sales are secondary and we'll get to it when we get to it." Paraphrased) The ass in charge decided to be a real jerk by sending the check USPS overnight mail and deducting the cost. Stupid?... yes. Not because he could have used regular mail and the cashed check would have been evidence of receipt, he was obviously 'sticking' it to me, but he was stupid because he could have used FedEx and it would have cost me even more.
Okay...the last paragraph was a rant but it felt good to "air my grievences" even though Festivus is over.