Eszett

A ligature combining a long s and a short s. Once common in English, the eszett, or sharp s, is now only used in German typesetting.

Unicode: U+00DF

Discussion:
long s + s vs. long s + z
Script Eszett
Uppercase germandbls is Coming to Unicode

Additional Articles:

From the ligature to a letter: SIGNA 9 - The Uppercase Eszett
A special issue about the history of the eszett in German only.
http://www.signographie.de/cms/signa_9.htm