You can find OmegaT here. A tutorial for beginners here. To understand what the OmegaT Project philosophy is, read this.

This group is an informal resource/support centre for OmegaT users.

OmegaT (r) is a free Computer Assisted Translation tool that runs on platforms including Windows, Mac OSX, and Linux.

Main functions:
 -TMX support
 -regular expressions based segmentation
 -fuzzy matching
 -keyword search
 -glossary support
 -Unicode support
 -spellchecking

Supported files:
Directly supported file formats: Office 2007, ODF, HTML, DocBook, limited XLIFF and PO support... See the documentation.
OmegaT supports TMX files up to version 1.4b (level 2 for most operations).

The OmegaT project and you!
You can translate OmegaT to a language not already available, but you can also do many other things to contribute back to the development of OmegaT. Read this.

Users:
FAQ: http://groups.yahoo.com/group/OmegaT/FAQ
IRC channel: #omegat on freenode (enter a nickname and hit "connect").

Developers / testers, requests for enhancements (RFE), bug reports:
Sourceforge page: http://sourceforge.net/projects/omegat
Bleeding edge: trunk tar ball (ant required to build)