You can use "redirection" on your website or redirection services like TinyURL (http://tinyurl.com/, they mentioned what you are looking for).
The best way is to buy a domain and redirect it to your link. You'll get better CTR and sales and it doesn't cost much to do... or you could put a php redirection file on a website you already own, then link to that php page.
Don't use link redirect services. Especially TINYURL, it looks spammy and many consumers avoid it! PLUS if the service goes down all your links go with it. Use .htaccess on your own domain instead. You'll get a shortened link that looks like yoursite.com/keyword Here's how... http://www.clickconsultants.com/using-htaccess-to-hide-affiliate-links
Ninja Affiliate is very good... but it's only for WordPress. If you cloak your links make sure you test them to be sure your cookies are not stripped out by browsers. .htaccess is a great way to go if you have only a few sites and are good at tracking things manually.