I see a lot of people using TinyURL.com and I gotta say it sounds a little scary. For those of you who know, anytime you access any website the IP address of your computer is available to that website's server. For example here's some information about you
That means when you go to that site to make a tinyURL they can record that information. On top of which, anytime any person you send that link to clicks on the link that person's browser will connect to TinyURL.com to get redirected to the actual site but while connecting, again, TinyURL can record both which link was used and the IP address of the person using the link.
In other words, TinyURL can track who made the link and every person who followed the link. There's all kinds of things that could be used for. A simple one might be advertising. If you like something maybe all your friends do as well. They could also check if one of your links was used by a known criminal or terrorist.
Of course the guy running it is probably a nice guy and is not doing that but it says right on his site that links are valid forever which means he's keeping at least some of that information. Who knows when someone will make him an offer to sell TinyURL.com and the buyer will decide how to use that info.
Of course any site can track the info shown above and most do. What they can't do is tell who told you about a particular page. When you use a tinyURL, TinyURL.com can tell who made the link and who it got passed to.
Note: I'm not a paranoid person. Part of this post is because I have friends that use TinyURL and those same friends worry about systems like Plaxo that track similar stuff. Also, Wired.com recently ran an article about TinyURL and didn't mention the possible issues, something that Wired normally seems hyper sensitive about.