The windows installer comes already with everything you need. It includes Slic3r, Skeinforge, Python and Pypy. Requires .NET 4 framework.
The Linux version requires a recent installation of the Mono framework. After unpacking the tar file you need to run configureFirst.
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The latest stable release of the Repetier-Host for Macintosh. Download and run the package to install. If you are using the new Mountain Lion, you need to allow installation of unsigned code!
Licence: GNU GPL V3
Version 0.91 with Configuration Tool
Source repository: Github
Older versions contain unfixed bugs and should be avoided when possible.