A BSD-derived commercial OS which used to be a good choice for web servers (before FreeBSD became more solid), but now has some disadvantages compared to FreeBSD and Linux, including:
- Costs too much. BSDI used to be fairly inexpensive ($100 or so), but the current version costs $995 ($2995 for a source code license!).
- Expensive source code.
Not as good as FreeBSD or Linux and a lot more expensive, but at least it's better than SCO.