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Gravit is completely free to use.
It was open source up until a recent release, however there are plans to open source it again in the future.
Pro Multi platform
Works on Windows, Mac and Linux.
Pro Actively developed
Con Changing rapidly
The fast rate of growth/change for Gravit can be a pro in that in means plenty of new features.
However, it also makes the product fairly unpredictable.
For example: In the past year, Gravit was open sourced, then closed sourced again.