Because short is better

Size does matter, the shorter the better.
Sure the spammers that hit your inbox disagree that :-D.

So where in the world this the-shorter-the better-idea exists?
The answer is in the world of Twitter, HelloTxt, Hictu, Jaiku, Plurk and those kind of service.
With the restriction of 140 characters per message, we need to be smart enough to fit the whole words we want to tell the words. That won't be a problem when there's no url address in it, but when there is, then we need a short url service.

Here is compilation of url shortening machines:
u.nu
xr.com (custom alias, edit-able, adsense integration, real-time tracking)
is.gd
tr.im (custom alias, password, real-time tracking)
nn.nf
to.ly
adf.ly (real-time tracking, pay-per-click, API, referral included in the link)
cli.gs (custom alias, password, real-time analytics, social media monitoring, geo-targeting, search engine friendly, tools, API)
2pl.us
bit.ly (custom alias , real-time tracking)
tini.us (custom alias)

There you are, a prove that short is better! :-D

UPDATED!!
fa.by (custom alias, password, bulk submission, cloaking, tracking, editable)