Choosing which Affiliate Broker(s) (also known as Networks) to work with is not an easy task. There are many things to consider such as which merchants they broker, services they provide, size of network, how they deal with Black Hats and spyware, Affiliate support and responsiveness to Affiliate requests. The list goes on and on but a there is a great resource to help you figure out what Affiliate brokers are for you. ABestWeb forum has the low-down on all of the major (and many off the beaten track) brokers and networks. Spend some time searching the forum and asking questions and you will quickly find out that where there is money, there are plenty of opinions.

Starting Out
If you are a Newbie, I would choose one Network with which to work. I would start with a Network with a good accredited Affiliate manager. Yes, there is industry accreditation for Affiliate Managers. I highly recommend shareasale.com. They have a reputation of honesty and integrity. (Unlike some of the larger Networks)

A list of some Affiliate Networks/Brokers
Shareasale.com - my favorite. They are know for their honesty and unwavering dedication.
Commission Junction - one of the biggest with thousands of very recognizable merchants to offer.
LinkShare - another large network with some very attractive brand names
Google Affiliate Network - Yep, Google is now in the Affiliate business
AffiliateFuture
Amazon
AvantLink.com
clickXchange
Clicksor
Filinet
incentAclick
LeadFlash
LinkMo
Market Leverage
Rextopia
ShareResults
Zanox