There are many ad and affiliate networks out there, that each try to cover a niche. Please list your favorites and give some reasons why you like them.
I use Amazon Affiliate Program on one of my websites, and find their aStore (iframe shop in your website) especially interesting.
But in general, when you have a product that you are writing about -- try going to the company webpage and check if they have a link to their affiliate program at the bottom.
Most big companies use CommissionJunction, some smaller ones use Google Affiliate Network ... but there are hundreds of those out there. The important bit is to read the terms of service and note what will you be getting from the clicks on the affiliate links.