Real Time Players

Exchanges:

Yahoo's Right Media - First Exchange, not RTB enabled...yet
Google's DoubleClick AdX 2.0 - First RTB enabled exchange
Microsoft AdECN - apparently coming out with RTB anytime now
AdBrite - Mostly comprised of long tail inventory
Contextweb's Adsdaq
Open X

Demand Side Networks (Agency Network/ Holding Company):

VivaKi (Publicis Group)
Cadreon (UMI)
Atom (Razorfish)
B3 (WPP)
Havas (AdNetik)

Demand side platforms/ Trading desks (companies focused on helping agencies create the technology to decision on and buy impressions in real time):

AppNexus - leaders int he space. Run by Right Media Founders.
Turn - Said to be powering UMI's Cadreon network
MediaMath - Said to be powering Razorfish's ATOM network
(X+1) - Demand side platform just launched in November
Aggregate Knowledge - Call theirs a "discovery platform"
Triggit - Don't seem to have anything to show right now

Publisher side Ad Platforms (companies focused on helping publishers get the most money out of their inventory, and allocate inventory to the highest bidders):

Ad meld
Pubmatic
Rubicon Project
Peer39

Others:

Fox Interactive Media - First to apply RTB to MySpace inventory back in 2007
AOL's Advertising.com - One of he first to develop real time bidding methods and currently one of the largest buyers on RTB basis