A one-inch hard drive in a CompactFlash-compatible form factor and interface.  The name is currently [May 2010] trademarked by Hitachi, though other manufacturers produce comparable drives.

http://en.wikipedia.org/wiki/Microdrive
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CobWeb''''''s from 2001:

http://www.storage.ibm.com/hardsoft/diskdrdl/micro/index.htm
See also: http://www.nvg.ntnu.no/sinclair/microdri.htm :-)