The best replacement for unity is gnome.

The best alternatives to unity are rated as following:
★★★★★  gnome
★★★★  udk
★★★  unreal
★★  unitarian
★★  uniformity
★★  diversity
★★  xfce
★★  mef
★  torque
★  cryengine
★  cocos2d
★  game maker
★  kde
★  blender
★  ninject
★  organizational conformity
★  libgdx
★  gnome 3
★  unreal engine
★  corona
  unity connection
  xna
  variety
  coherence
  cinnamon
  gnome shell
  unitarian universalist
  objectbuilder
  structuremap
  trinity
  greenville
  unitarian church
  cultural diversity
  opengl
  xsan
  unanimity
  windsor
  fragmentation
  harmony
  gamesalad
  genuine opposition
  oneness

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

Other search terms related to unity and their ratings are:
★★★★★ diversity
★★★★ scripting
★★★★ coding
★★★★ git
★★★ compiz
★★★ visual studio 2013
★★★ svn
★★★ github
★★ c++
★★ oculus rift
★★ blender
★ asp.net mvc
xamarin
You have in mind a companion for unity and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of unity in all years. Click on graph for more.
unity
gnome
udk