Is there an online Translitterator anywhere?

I need not a translator… but a “Translitterator”.. so I can copy/paste texts in languages that do not use Latin fonts.
Or even, better.. is there a software I could download for this same purpose?
(fro instance, I want to write a message in Greek to a friend of mine..since I know how to write in Greek all I need is a means to replace the “O” with the “Omega” font.