Mask and Track Affiliate Links in WordPress

This WordPress plug-in makes it easy to mask your affiliate links:

It also has click tracking and CJ hooks.

Another plugin that does this is Urban Giraffe's Redirection plugin. (just google the name if you want to find it). With that plugin, you can use it to do any kind of redirect and it does keep track of how many times the redirect is enacted so it will give you rough stats as well.

[Edit] Also, you can set the Redirection plugin to automatically create a redirect any time the post slug is changed, which is a nice feature.