Hi Steller,
I use a mac, you have a couple of options.
1) GPSResults (native OS X)
www.gpsresults.com/2) GPS Action Replay (java)
gpsactionreplay.free.fr/3) Use an emulator or virtual machine with windows, and run RealSpeed (
http://www.intellimass.com/RealSpeed/Index.htm)
4) There is a new web-based calculator which is showing real promise, I don't fully recommend it yet but it looks like it could be a viable alternative very soon:
www.ka72.com/When you join the challenge by sending Hardie a fax, you will be able to log in. Once you're logged in you just type in the six numbers and a bit of a story about how your session went. It's up to you to make sure that your numbers are accurate.