Tracking visitors after they leave my site

I apologize if this has been discussed previously but I couldn't find any answers.

I'm looking for a way to track where my visitors go (which sites they end up surfing to) after they exit my page. This would give me a way to target the appropriate affiliate program to use. I know that compete.com has "referral analytics" service that will do this, but it cost $500/month. I'm looking for an alternative to this, such as a tracking cookie or something like this, if it exists. I know that I can see which page on my site that they exit from using google analytics, but that isn't really very useful data for me; at least not for the site I want to use it for.

Any suggestions would be appreciated!

There's a few ways.

1. Spyware/Adware on their computer
Good luck with that.

2. Quantcast. You place a tracking pixel of theirs on your site. Other major sites across the interwebs use it as well.. They have some reporting features that will give you a list of sites and some demographics.