I'm pretty shocked there isn't an easy and trusted solution for this, but I've yet to come across it if it's out there. Wordpress and Joomla both have Drupal badly beat for this functionality I'm afraid.
Basically many affiliates want to link to sites without their users seeing the links, or knowing exactly where they're going to end up. It also helps with SEO.
We need a Drupal module that will add masking, redirecting and tracking of outbound links. Ideally it should work with the Link module (http://drupal.org/project/link) as that's what most people use to save URL's. Says nearly 70,000 sites use that module.
More information in this thread: http://drupal.org/node/821408