There are many services that will do this for you (I use the first, but the others are seen extensively):
And if you insist, there is even one that will put an ad at the top of the target page and earn you some pennies (i don't like those, so i won't encourage you by providing the link). These services have javascript bookmarklets that sit in your link bar and look like any other link, except that when you click it, the page you are on is passed to tr.im (or wherever) and you get the short URL that corresponds to the page you were on. Of course, if you are using NoScript for Firefox, this may not work -- you may have to temporarily allow scripting on the page.
However, some people have started using these short URLs to hide their real purpose -- give you a nice come-on with a shortened URL that takes you to a web page that (if you saw the URL) you wouldn't ordinarily go to. To your rescue are two services (free) which will show you what any given short URL expands to. Depending on what you see, you can choose to follow the short URL or not. The two that I know of are: