What are the best free CAT tools which I can download on my computer to use in work ? Thanks in Advance :)

I've tried Wordfast. It's pretty good for Microsoft users, but it's will make you computer heavy and slow. :(

I hope there is still readers or followers of this topic, anyway, there are some free CAT tools but the one I tried and succeeded with is OmegaT.  It comes in open source and provided free of charge.  One can find OmegaT download links and how to use it for your translation tasks at the official website of OmegaT.