realXtend

Minimum system requirements for realXtend Viewer:

Windows XP/Vista
A graphics card with shader model 2.0 support,
128 MB graphics card memory and up-to-date OpenGL drivers
512 MB system memory
Pentium 2 GHz / AMD 3000 processor

realXtend still not yet released a version for Mac. It's possible to use installed Boot Camp with Windows XP on a Mac, or Parallels, but it may not play nicely with video-card heavy applications like Second Life. There is also a tutorial about capturing SL machinima using FRAPS and virtual Windows on Mac.