I think there are plenty of Windows apps to do this, but on the Mac, I have successfully converted some Real Video files into DivX avi movies which can be played through Quicktime (with the DivX codec installed) using ffmpegX
There additional instructions for installing the additional files needed for ffmpegX to handle Real content.
Even with the additional libraries only certain older Real formats can be converted.
It takes some experimentation to get it working....
Here is the ffmegX site: