NSFW.in is designed for sharing links that are Not Safe For Work. Its unique feature is a warning page, in case the person you're sharing your link with doesn't want to see it after all.
Bit.ly is still one of the best link shorteners around. It scores points for tracking the traffic to your shortened link, and letting you see if anyone else has made a bit.ly link to the same URL.
U.nu is a fairly recent entry with a super-short base URL, and the added bonus of generating links that are easy to read out loud to someone or type out from scratch.
Awe.sm is now offering to create a custom URL shortener for its clients, based on its own service that's not too shabby either.
Tinyarro.ws uses unicode symbols to make its short URLs even shorter (and kind of cool-looking).
SHUURL secures your shortened links by showing users a thumbnail of the site you linked to before they click through.
1link.in opens multiple sites with one shortened link. Users see a list of the links that are included, with the option to open one at a time or open all of them in tabs.
Tr.im is my personal favorite of the bunch. It's on the shorter side of short URLs, it tracks stats, and it lets you post your links to Twitter with one click.
BurnURL offers social sharing options for your short links, so you can easily Twitter, Facebook, Digg or Reddit them.
The DiggBar was the source of much controversy when it first launched, but it continues to be one of the best ways to share your Digg links.
TinyURL is considered the original link shortener, and still one of the best. Aside from being the default for Twitter, it's also kept up with the new kids by adding the ability to customize your links.
ceSnipURL is a shortener for Windows Mobile.
Su.pr is StumbleUpon's link shortener.
And, for a counterpoint to all of this shortURL madness, take a look at why Download Squad's Lee Mathews is sick of URL shorteners.