Complex sentence: The company, founded in 2000, was orginally most notable for their support of open-source software, namely their SlimServer software which their products at that time all depended upon, and is still available as a free download and modification by any interested developer.
Split into simpler sentences: The company, founded in 2000, is notable for their support of open-source software. Their SlimServer software, which their product depends upon, is available as a free download and modification by any interested developer.