A partial list of who is acquiring, licensing, and enforcing patents:

Acacia Research Corporation
IPotential
Inflexion Point
Altitude Capital Partners
Ocean Tomo
Rembrandt IP Management
NW Patent Funding Corporation
Intellectual Ventures
Gathering2.0 (online community for monetizing IP assets)

Articles:
Patent Attorneys, Clients Banking on IP Brokerages (law.com)
Patent Pirates (forbes.com)