Best International Affiliate Advertising Networks

LinkShare.com
Commission Junction
Offersquest
Affiliate Fuel
Share-A-Sale
ClickXchange
Refer Back
Casino Coins
Dark Blue
Lead Hound
Direct Leads
Fine Clicks
Clix Galore
Commission Monster
Buy.at

Best Affiliate Programs In The United Kingdom
Tradedoubler
Affiliate Future
Affiliate Window
UK Affiliates
Affinity Arc
BeFree

Others Running Their Own Affiliate Programs
There are still a large number of companies running their own programs using their own affiliate tracking software. There are harder to manage than those which use affiliate networks and disputes will be handled between the two of you. Still, if you can't find the companies you want on any of the networks then these are some of the top alternatives.

Amazon
All Posters
eBay

Still looking? Here's a tip. If you are looking for a particular type of affiliate program to partner with then hit your favourite search engines with search terms like "*keyword* affiliate" and variations of that. For example, say you have a webpage devoted to your collection of collectable frogs and are looking to partner with a shopping site which sells colectable frogs, porcelain frog miniatures etc. Key in things like "frogs affiliate" or "collectables affiliate program"...that kind of thing.