NetBSD

NetBSD is a free (as in freedom and as in free beer) OS with focus on portability. It's based on 4.4BSD and is one of the oldest descendants of it.

NetBSD's logo is an orange flag

Why use it?

• NetBSD is more open to experimenting with, trying out and configuring things, rather than the more conservative OpenBSD. This doesn't much such is impossible on OpenBSD, but the NetBSD people tend to be more supportive with this stuff, while you probably fall on deaf ears with the OpenBSD people, if you screw up some custom configuration of your system.
• NetBSD has pkgsrc, which is a portable and stable package manager, which includes a wide variety of binary-packages as well as can compile from source.
• Everything in NetBSD is meant to be as portable as possible. Not only does this mean, that you can run NetBSD on a lot of processor architectures, but also that the code is cleaner.
• NetBSD is not Linux and thus doesn't have many of the cancers, that plaque Linux.
• The documentation is great. There is "The Guide", the Man pages and a small Wiki.
• It uses plain text, 72-char mailing lists instead of a forum,

Reasons against using NetBSD

When you need...

• Wine; Wine does exist on NetBSD, but it's not nearly as well supported as on Linux.
• Steam, IDA Pro or any other proprietary software, that runs only on Linux.
• long battery time. There is the powerd daemon, but you shouldn't expect much from it. Expecting 1-3 maximum hours on most consumer-grade hardware is reasonable.