Many of us use URL shortening services, where you feed the service a long URL (web address) and it gives you a much shorter URL which when clicked redirects to the "real" long URL.
This is useful because long URLs in emails often break across lines and won't work unless the recipient reassembles them manually, which is a pain.
Also, short URLs obviously take up less space where it matters, notably Twitter with its 140 character limitation (and Twitter do automatically shorten your long URLs for you).
A major issue with URL shortening services is that the short link goes to their website, and they then redirect the user to the real intended destination site. So if the URL shortener site is down or stops working or disappears, e.g. because the service went bust, the short links won't work anymore.
Fortunately, recently a bunch of URL shortening outfits got together to form a new service, 301works.org, which will be administered by the illustrious Internet Archive - see the 301works blog post of 11 November 2009 and the Internet Archive's post. The idea is that:
"Participating companies will provide regular backups of their URL mappings to the 301Works.org service. In the event of the closure of a participating organization, technical control of the shortening service domain will be transferred to 301Works.org in order to continue redirecting existing shortened URLs to their intended destinations."
So when you use a URL shortening service, it's safest to use one of the participating companies.
The companies taking part so far (see the 301works site for the latest list, which looks set to be growing fast) are: