This software was used in APIO'08. It currently has mainly two components: the web submission system and the grader. The web app uses Ruby on Rails; while the grader was written in plain Ruby. It also uses the sandbox developed by Martin Mares as part of the MO Contest Environment.
We have moved to Gitorious.
Currently, we are combining the two main components (i.e., the web interface and the grader) so that all administrative tasks (including problem uploading and grading) can be done by web interface.
If you can read Thai, see this for installation.
For general specification (a bit out-dated) of the grader see its specification. For installation, see this for web interface (again, quite out-dated, as for now an easier way is to use Phusion Passenger), and this for the grading system (or judge) (also out-of-date).