OmegaT is a free Computer Assisted Translation tool that runs on Windows, Mac OSX, Linux etc.

OmegaT is here. A tutorial is here. Full documentation in several languages can be found here. The OmegaT Project philosophy is defined here.

Main functions:
-TMX level 2 and glossary support
-sentence segmentation
-fuzzy matching
-powerful search
-spelling and grammar checking
-Office 2007, ODF, HTML, DocBook, PO, limited XLIFF support

OmegaT and you!
You can make a donation. You can translate OmegaT to your language. You can also contribute 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").

Development / tests, requests for enhancements (RFE), bug reports:
Sourceforge page: http://sourceforge.net/projects/omegat
Bleeding edge: trunk source (ant required to build)