Animanga.com - Yann has some terrific cels for sale, and is very pleasant to deal with. His cels are mailed from Switzerland, however, which can be twice as pricey as shipping to the US from Japan. 

Anime Assault.com - They're fine. Odd way of handling surcharges, though.

Anime Barn - Very friendly dealer, communicative, and quick.

Anime-cel.com - A wonderfully low-priced cel vendor. Sometimes, they get some goshdarned nice cels on their site, but for the most part, I could pass. They're worth keeping an eye on, though-- if you act fast, you can sometimes get a real bargain. They're very prompt in their turnaround time.

Anime Chaos - Very popular site, moderately good turnaround time, wide selection.

Anime Dep - Sato's deputy bidding service. In case you see a terrific cel wind up on Yahoo! Japan, and the seller won't ship to the US... you can use a service like Sato's. He will bid for you, and you'll send him the money, plus an extra percentage to cover his costs (ie, shipping it around Japan, then to you; reimburse him for his time) and he'll send you the cel. I've used him a few times, and have had my ups and downs, but overall, it's been a good experience.

Anime Kingdom - I wasn't too thrilled at first, because they neglected to confirm my order, and a week or two later, I phoned myself to make sure the cel was in stock before sending off payment. But it seems that they were having email difficulties, and they were quite friendly and polite. They updated their right after my order arrived, and the cel shipped out very quickly.

Celmart.com - Hashi had some terrific cels on his site, before I got a little more serious about collecting. There are several things that I've bypassed from him which I wish I hadn't. ^_^  It used to be a terrific place for Recca cels, especially, but recently, all his good stuff's been going straight to Ebay, and his prices have inflated somewhat in the last year or two. *pout* Still, I keep an eye on him in case a gem shows up. He has a very good turnaround time.

Cels.org - A very comprehensive list of cel vendors, and part of the Animanga site! The vendors are rated, and it's quite a handy place to begin a search when you're interested in investigating new territory.

Ebay.com - Also known as Evil-bay. ^_^ But it all depends...My experiences, as far as cels have gone, have been good. It's wonderfully convenient, and you get a good diversity of cels coming up on the block. A lot of nice cels of good characters can be had for less than a usual cel vendor... which is immensely helpful, when, say, you and hubby are both in grad school. *cough cough* But if you get into a bidding war... *shakes head sadly* But you take the good with the bad, and see what comes up!

Pocket Partners - If you buy from Pocket, you might want to get in touch with Animiko and get a group going. You'll save on fees and shipping.