Conference Management systems are web applications designed to make the lives of conference organisers easier. They usually include features such as registration, payment, paper submission and review, scheduling and publishing of announcements.

Since Wikipedia is apparently not a place for such a "spammy list," I figured I should at least post this here:

NameUsed byLicenseProgramming Language
A Conference ToolkitYAPC::EuArtistic LicensePerl
ConManUtah Open Source Conference, Texax Linux FestGPLPython (Django)
Open Conference Systemsvarious academic conferencesGPLPHP
OpenConferenceWareOpen Source BridgeMITRuby on Rails
PentabarfDebConf, FOSDEM, CCCGPLRuby on Rails
SCALEregSCALEGPLPython (Django)
Zookeeprlinux.conf.auGPLPython (Pylons)

If you know of any other Open Source systems, please leave a comment!