Gustavo,
I think your question has prompted the forum owner to subscribe to the OmegaT user group, which is a good thing, but let me give you a few links that will help you get a better idea.
First of all, the wikipedia article that the OmegaT team wrote a few months ago:
http://en.wikipedia.org/wiki/OmegaT
Then the user group page (archives open to non subscribers):
http://groups.yahoo.com/group/omegat
As written by IUS (basically a copy-paste from the user group top page), OmegaT is a free CAT tool.
What it does not do is allow a translator to work on Trados jobs though.
I've worked with OmegaT as a professional translator for as long as it has been on the CAT tool market and I have not used any other tool, mostly because I work with a Mac and Trados and the likes are Windows only.
If you have any specific question, don't hesitate to ask, either here, or on the user group.
Regards.