The best replacement for osx is linux.

The best alternatives to osx are rated as following:
★★★★★  linux
★★★★  windows
★★★  ubuntu
★★  windows 7
★  vista
★  osx x86-64
★  osx server
★  ios
  windows vista
  longhorn
  os9
  microsoft xp
  freebsd

Your alternative for osx is not listed? Suggest!
Speaking about osx, one probably shouldn't forget about vmware.

Other search terms related to osx and their ratings are:
★★★★★ vmware
os9
You have in mind a companion for osx and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of osx in all years. Click on graph for more.
osx
linux
windows