FreeBSD, as the name suggests, is a totally free Operating System, and in my opinion, it's the best and most stable operating system available today.
FreeBSD is used to run Yahoo (see here for more information) , and the servers of the Apache project.
On September 29th, 2000, the record for the largest download from the Internet in one day
from a single machine was achieved on a machine running FreeBSD. The site,
ftp.freesoftware.com surpassed the two trillion bytes (two terabytes) milestone in under
24 hours.
The server is "only" a Xeon/550 with 4Gb of memory, and 400GB of Raid 5 storage, and
typically runs with 6000 simultaneous users every day.
[ link ]