bit.ly – URL Shortening Service
bit.ly is another web service like the popular tinyURL. You paste the long URL in the text box at bit.ly, optionally set a custom keyword and click on Shorten button to get the shortened URL.
For example, let’s say that I am researching on water purifiers and came across a page whose URL is http://www.eurekaforbes.com/products/healthylife/wpurifier.php. Now I want to be able to go to this link in the future without having to bookmark and by just typing out an URL. I can go to bit.ly, specify ‘aquaguard’ as the keyword and it will generate this URL which points to the same page: http://bit.ly/aquaguard.
bit.ly doesn’t yet have all the useful tools like a Firefox add-on or integration in Twitter clients like twhirl. A browser bookmarklet is available though. But with a good API to shorten, expand and extract URL info, I am sure bit.ly is going to replace tinyURL from its #1 spot.
Visit bit.ly.

One extra feature that Identi.ca has that Twitter currently doesn’t is Instant Messenger support. Not only can you be notified via your GTalk or Yahoo! Messenger or a Jabber client, but also post your updates from within IM clients. For that you have to first add 

