"Few things make me so quick to anger as ow.ly links." - @ourmaninchicago

You've seen them. Those Tweets with URL shortened links from Hootsuite (ow.ly), Stumble Upon (su.pr), and Friend Feed (ff.im).  If you've clicked one before then, like me, you recognize that they are a manifestion of purest web Evil.  Why?  They frame the site you are posting and put an obnoxious toolbar for their product at the top.  Reasons to despise this are:

1. It's a security risk (part 1) since you can't see the real URL of the site.  The only thing that shows up in the URL bar is ow.ly/xxYYzz or whatever.  While all URL shorteners have risks, this is particularly bad since you don't know if the actual site underneath is genuine.  Sure, it might look like the New York Times, but what if is really a scammer who prompts you to log in and captures the username/password combo that you happen to use for everything?  Not good.

2. It's a security risk (part 2) since they can track your surfing.  Unless you somehow type in a new URL or hit a bookmark or something, clicking links only reloads the child frame, so the URL bar stays in place.  This not only perpetuates the risk above, it also theoretically lets Hootsuite track your every click.  Not Good.

3. It turns you into a flack for their product.  Conveniently, if you post something using this, you are now helping to advertise and promote the product to other people.

4. It promotes further viral spreading of these obnoxious links.  The toolbar contains icons to let the consumer directly re-tweet it - using ow.ly, et. all. of course.

Presumably most people aren't consciously using these products as URL shorteners.  Rather, they are using the tools as social bookmarking and one of the many fine "benefits" that comes along with the product is this evil framing link wrapper.  Be careful Tweeple!

A couple more URL shortening services to avoid: Digg and tr.im

As for Digg, I actually think it's a great place to find cool stuff, but not for sharing with others via Digg links.  These hijack the original link and, even worse than the above, send you to a landing page, not the original site.  Dat's not nice.

tr.im has a cute name, but the service is flaky and already announced they were going out of business once.  So unless you don't want your friends to see this:

avoid tr.im.

Save yourself grief.  If you want to shorten links, just use bit.ly or tinyurl.com.  No guarantee they won't turn obnoxious or go out of business either, but the others are already there.