Jorpho over at the HOTU forum has done nice work gathering a list of software vendors. Since the list is very handy when looking for rare titles or cheap offers, thought the list should appear here as well. Besides, there must be good sites not listed yet, so you fellows can add more to it.
I've marked the ones I have used myself with good success with an asterisk (*).
Also, (E) means the company ships to Europe.
The Great List of Vendors:
Amazon.com * (E)
E-CDRom (E)
Cyber City Software (E)
5 Dollar Software (E)
StarVector (E)
Xplosiv.net /
Empire Interactive Store (E)
CDRomShop.com (E)
GreatDealsSoftware.com (E)
BigDaddySoftware.com (E)
CrazyApe.com (I think) (E)
Focus Multimedia (www.fmselect.co.uk) (E)
E-DealsUSA.com (E)
Sold Out Software (www.sold-out.co.uk) (E)
Overstock.com (E)
SimplySoftware.net (E)
JB Software (BestSoftwarePrice.com) (E)
Play.com * (E)
Game.net (E)
EBGames.com (E)
GameStop (E)
NothingButSoftware.com (E)
Gameplay.co.uk * (E)
Purplus Software (E)
GamesWharehouse.com.au (iffy) (E)
C-Deals.com (very limited stocks) (E)
Some not-so-well-reputed ones that might come handy anyway:
The Software Society (E)
Camelot.ca (E)
PlayingGamesInteractive.com (Down?)
GoGamer.com
CDON.com (currently only in the nordic countries) *
(It's actually the first thread opened by me, this one.)
EDIT: Added Purplus Software.
EDIT: Added cdon.com