Simple URLs plugin for WordPress is a complete URL management system that allows you create, manage, and track outbound links from your site by using custom post types and 301 redirects.
Simple URLs adds a new custom post type to your Admin menu, where you can create, edit, delete, and manage URLs. It stores click counts in the form of a custom field on that custom post type, so it scales really well. By avoiding page based redirects, which is the current trend in masking affiliate links, we avoid any issues with permalink conflicts, and therefore avoid any performance issues.
Plugin homepage: http://www.studiopress.com/plugins/simple-urls