Affiliate Networks

An affiliate network acts as an intermediary between publishers (affiliates) and merchant affiliate programs. It allows website publishers to more easily find and participate in affiliate programs which are suitable for their website (and thus generate income from those programs), and allows websites offering affiliate programs (typically online merchants) to reach a larger audience by promoting their affiliate programs to all of the publishers participating in the affiliate network.

The big 4 affiliate networks

Commission Junction
Probably the most popular affiliate network, Commission Junction also has its share of critics. Finding and joining an appropriate affiliate program is easy. CJ also makes things easy for affiliates by consolidating all payouts into one check. It displays EPC (which to CJ rather oddly means "earnings per 100 clicks", not earnings per click). CJ is owned by ValueClick, which merged with Be Free.

LinkShare
LinkShare has more than 600 merchants in its affiliate network. Choose from categories such as books, music, flowers, fashion, computers, etc. You'll find some big name companies here, including Dell, Nordstrom, Target, Disney and American Express. LinkShare is owned by Rakuten, which also owns the controversial OneCause toolbar.

ClickBank
Offers a huge array of downloadable products, mainly ebooks. Because it is cheap and easy for merchants to join, the quality of the products provided varies enormously, from very good to downright awful. It's a popular place to search for overlooked niches.

Google Affiliate Network
Highly professional site with some well known brands. Clients include: America Online, Blair Corp., Bose, CompUSA, Eddie Bauer, HP Shopping, PC Connection, RedEnvelope, Verizon Wireless, and more than 100 others.

and some others:
ShareaSale - highly recommended
Affiliate Window
Click2Sales
MaxBounty
Pepperjam Network