The best replacement for slic3r is skeinforge.

The best alternatives to slic3r are rated as following:
★★★★★  skeinforge
★★★★  cura
★★★  kisslicer
★★★  makerware
★★  replicatorg
★★  simplify3d
★  sfact
  netfabb

Your alternative for slic3r is not listed? Suggest!
Speaking about slic3r, one probably shouldn't forget about pronterface.

Other search terms related to slic3r and their ratings are:
★★★★★ pronterface
You have in mind a companion for slic3r and it is not yet here? Propose!



Google Trends

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slic3r
skeinforge
cura