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 February 17th 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 (January 20th). 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
244 (258)		@istockhelp
529 (532)		@fotolia
752 (777)		@dreamstime
989 (1036)		@shutterstock
2170 (2353)		@123rf
4031 (4082)		@bigstockphoto
9183 (8878)		@crestock
13140 (12737)		@veerupdate
18301 (18821)
21715 (25269)		@pixmac
26093 (26212)		@photocase
28444 (29013)		@The3dStudiocom
29164 (69079)		@deposit_photos
34427 (n/a)		@dlstl
36133 (36961)
54768 (57795)		@panthermedia
68171 (72282)		@snapixel
68361 (65457)		@cutcaster
74764 (90494)		@moodboardblog
119k (.de) (107k)
132k (195k)		@andaluciaimagen
132k (169k)		@mostphotos
147k (136k)		@yaymicro
383k (424k)		@vivozoom
390k (412k)		@polylooks_com
466k (488k)		@clustershot
873k (818k)
>1.6 M		@arcticstock