Of primary importance:

PURLS

Of secondary importance:

http://metamark.net/ (which uses xrl.us)

the WWW::Shorten perl module

Also:

http://babyurl.com/

http://www.makeashorterlink.com/

http://1sl.net/

http://shorl.com/

http://snipurl.com/

http://tinylink.com/

http://tinyurl.com/

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Use Metamark to make a long URL shorter: