Ogg Vorbis was specifically created to be a free audio format, so I should hope you find a free converter! 
If you're looking for customizability, go with Foobar2000. (For a guide to set it up with the highest-quality Ogg Vorbis encoder, scroll down to "Encoding from Files" in this post.)
If you're just looking for a quick, easy solution and aren't picky about the sound quality, I'd go with dbPowerAmp.