Apache running on Linux.
As far as the database portion of our systems, we run MySQL using InnoDB to provide transactions, row locking, hot back up for MySQ, all without compromising the high read performance of MySQL.
We currently support a number of open source projects in order to leave the world a little better than we found it. For example we recently made a donation to the Perl Foundation. Perl is a one of the core languages of the net and most of our projects tend to get written in Perl.
We also contribute to the Xao Web Development Suite currently maintained by Andrew Maltsev. We are currently extending the suite to include a new Forum object and blog platform.
Whatever technology we continue to develop, we will endeavor to open source those elements we feel that will benefit the community at large. Once again, trying to leave the world in a better place than we found it.