Here a list of the main photographic microstock agencies. I use referral/affiliate programs for most of these stock sites (the ones with msp logo in the badge corner).

The number on the left is the 3 month avg Traffic Alexa Ranking, updated on January 20 2010 (combination of average daily visitors and pageviews over the past 3 months, by www.alexa.com). Between brackets you can find the previous Alexa rank takeover (December 21). These numbers are not evidently related to the revenues volume, they just give a roughly idea of the market positions.

Alexa Rank	Stock Agency	Twitter account
258 (262)		@istockhelp
532 (522)		@fotoliausa
777 (798)		@dreamstime
1036 (1065)		@shutterstock
2353 (2356)		@123rf
4082 (3889)		@bigstockphoto
6764 (6786)
8878 (8781)		@crestock
12737 (13059)		@veerupdate
18821 (20558)
25269 (30426)		@pixmac
26212 (24686)		@photocase
29013 (n/a)		@The3dStudiocom
36961 (35065)
57795 (56189)		@panthermedia
65457 (69038)		@cutcaster
69079 (86332)		@deposit_photos
72282 (69899)		@snapixel
90494 (93860)		@moodboardblog
107k (.de) (n/a)
136k (136k)		@yaymicro
169k (174k)		@mostphotos
195k (n/a)		@andaluciaimagen
412k (424k)		@polylooks_com
424k (391k)		@vivozoom
488k (396k)		@clustershot
818k (908k)
>2 M		@arcticstock