1) Federation Models:
https://www.federationmodels.com/
They've got quite an extensive list of Resin and Styrene kits, and not just Star Trek. They are honest, they will go above and beyond the call of duty to insure you're not overcharged, and will reimburse you if your postage and handling was less than expected.
2) Starship Modeler
https://www.starshipmodeler.com/
or
https://www.starshipmodeler.biz/shop/index.cfm
My second favorite place to purchase models. Often he will list models for sale that Federation Models doesn't.
3) Starfleet-Models (Now Defunct).
4) Starship-Models
https://www.starship-models.com/
A german outfit, I've sent off an order for a couple of items. I'm waiting to see if they do deliver. (March 13, 2013).
Update 03/21/13, they came through with my order, and I'm happy.

5) JT Graphics
https://jt-graphics.com/MK_JTG.html
Very nice selection of some Star Trek, Babylon 5, Battlestar Galactica, etc. This site contains several companies that produce resin kits, but also has a wonderful lineup of Decals. Check it out. I've purchased several items from here. No complaints yet.
6) Fantastic Plastic
https://www.fantastic-plastic.com/
A wonderful site that is both Virtual Museum and has a catalog of kits that include science fiction, real spacecraft, and concept craft. I've purchased an item or two. The last one was the U.S.S. Emmette, the ship that appeared for only a couple of seconds on Enterprise during the intro song.
7) Cult TV Man
https://culttvman.com/main/
The Hobbyshop offers not just models, but a large number of other items. And it spans not just science fiction but into horror as well. I've purchased a couple of items from here.
8) Monsters in Motion
https://www.monstersinmotion.com/
A large selection of items from the Movies and TV, also sells international kits and such. I'm awaiting an order for a Nostromo as of now (March 13, 2013). The Nostromo arrived on March 18, 2013. And my nipples explode with delight!
9) Round 2 Models
https://www.round2models.com/
Why buy something at an inflated price off of ebay when you can get one from the producer? I've purchased the TOS 1701 kit and accessory kit from here.
10) Don's Light and Magic
https://www.dlmparts.com/home.html
A good source for some extra parts and lighting kits for Star Trek. Several items from here adorn my 1:350 Refit Enterprise.
11) Trek Modeler
https://www.trekmodeler.com/
Do you like lighting up your kits? My 1:350 Enterprise Refit was lit up using a couple of this one's kits. And it was well worth it.
12) Modular Models
https://www.modular-models.com/
Maker of many fine 1/1000 conversion parts.
Update November 2016- it appears that even though his website is running (and taking money), he himself isn't answering the door (so to speak)...