The 500th committer is dan-btown, an admin of several sites running Tiki.
The list of 500 committers is available on SourceForge and while there are Free/Open Source projects like the Linux Kernel with thousands of contributors, Tiki is the largest one that is hosted on SourceForge.net. If you'd also like to join the action, please check out how to get commit access. Thank you to SourceForge.net for hosting us all these years!
Please listen to the podcast on the topic: http://sf.net/blog/podcast-tikiwiki
This is yet more demonstration of the scalability of Tiki's "wiki way" development model. The Tiki community strives to continue to lower the bar to participation with projects such as the translation server, at i18n.tiki.org, which permits translators to contribute in context, through the web interface.
For more historical perspective on the early days of Tiki, check out when SourceForge awarded Tiki Project of the Month, when it was less than a year old.