Part of the PGP/GPG Public Key Infrastructure (PKI).
An server that cryptographically timestamps and signs data to enable the submit to later prove that certain data was in their possession at a certain point in time.
It implementation, a TimeStampServer is actually quite similar to a RobotCA, and similar attack models seem likely.
2 pages link to TimeStampServer:
http://www.itconsult.co.uk/stamper.htm