Just for the record: the RAR format is no Microsoft proprietary technology. It's more like a better-compression alternative to the omnipresent Zip format. (www.rarsoft.com)
If there are no open implementations of it, it's proprietary.
There is a Linux command-line binary, but it's a "trial" version you're expected to pay for. Which I can't see myself doing, especially since there's a free BZIP2 compressor which can't be much worse.
The RAR download comes with a free uncompressor, unrar.
You can get the source code to a decompression library for it from the website.
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