Ever since social networks came to the mainstream, “sharing over the internet” has been acquired new dimensions. Sharing interesting links has now attained the status of a hobby. People needed neat & simple ways to do this; and there came URL shorteners (services that creates a shorter URL from a given long URL). There are hundreds of them on the web (my favourites being lin.cr & is.gd. There are many more, like, moourl, tinyurl, url.ie etc.)
But, you should be aware of their malicious potential. Any service could be misused. The case is no different for shortened URLs. Malicious links could be contained in such shortened URLs, whose target link is unknown to the user. When tried to shorten an already shortened URL, the service said: “This usually means you tried to link to is.gd itself or another URL shortening site. Links to these sites are disabled to stop spammers hiding abusive links behind a chain of shortened URLs.”
Look at some of the possible threats:
The link associated with the LOGOUT button in Orkut is http://www.orkut.com/GLogin.aspx?cmd=logout. Now that link could be converted to something like http://ab.cd/ef Clearly, clicking on that link will log you out of the site. This is only a simple example. Think of situations where, these could be used to (yes) DELETE your account, gain ownership of communities, copy/modify your personal information, perform restricted operations etc.
Now you know how dangerous these could get. (It is when I wrote this post, I understood why Orkut and several other social networks delete those shortened URLs posted by its users!!)
But, these are extremely useful services (especially at twitter, where there is 140-character-limit) and shouldn’t be ignored because of such threats. I just wanted to say, be careful while following a shortened URL; you never know where it takes you or what it does!!
Update: I came across a GreaseMonkey Script that Restores a TinyURL and shows the original url in a tooltip on mouseover.
Update: Here’s a script that Automatically changes all tinyurl links to preview.tinyurl, so that you know where they are trying to send you.
If you want more URL shorteners than I listed, you may head here to read a long list of URL Shorteners: http://lin.cr/nu
(Don’t worry. This link is SAFE 
)
Lin.cr is created by Aalaap Ghag
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Very true, actually used this once
on a friend gave him a logout link and told him it’s some new cell phone check it out and he kept on saying arre yaar, I am getting loggged out, I said you have got a virus this and that scared the shit out of him 
Wow! Manan have just written ‘Scout’ Office 2007 Commands
Hmm, you know I never quite thought of this aspect of Link shorteners. Neat
Wow! Sathya have just written I wanna listen to music!
This could actually be the death of Twitter. If you look at TwitterBuzz.com, a lot of the links are bad or ads already. Do you know of a service that provides a tinyurl-like shortened URL, but shows you the link, or info about it, before taking you to it?
Wow! Dave L have just written Skip The Honeymoon? Engagement Straight to Divorce.
Sadly, I couldn’t find any service that shows the target location it is linking to.
Infact, DIRECT REDIRECTION is considered as a GREAT FEATURE on those sites.
So, for the time being, we can only be cautious while using those !!
@Manan
Many of my friends had this trick played on them and consulted me.
@Sathya
This is indeed an important aspect to keep in mind while using those links.
@Dave you can try using http://canyouopenthis.com this shows if the site is up or down and provides a snapshot of the site as well.
@ Dave & @Sathya & @*ALL*
As sathya pointed out, you can get to know where the link will take you by using services like SNAP which loads a preview of the target page in a small frame.
And herez a graphical way to ping a server:
http://downforeveryoneorjustme.com/{WEBSITE_NAME}
I’ve started switching to http://preview.tinyurl.com/5zv7nk — it’s only 7 characters more, though would be nice if a service standardized on this. Eventually I’d expect they would have to, as the hidden redirect aspect gets abused more often.
Wow! Dave L have just written Google: So powerful, even their jokes generate bandwidth
I spoke too soon - http://peek.snurl.com/28pff is an even URL shortener, I just didn’t notice the “peek” button on their control before.
Wow! Dave L have just written Google: So powerful, even their jokes generate bandwidth
Cool.
I wonder why I missed them !!
But, this could be done only to TinyURL and SnURL, right??
There are many more URL shorteners out there.
We might ask Aalaap (creator of lin.cr, linkbun.ch, canyouopenthis.com) to find a way around this if he can.
But, for the time being, Use http://CanYouOpenThis.com