FreeBSD is used by leading web sites such as Yahoo! and Internet Service Providers like Best Internet, and it is widely used for Internet domain name and mail servers. Because FreeBSD is maintained by a core group of developers it tends to be more consistently designed and implemented than Linux.
Both are built on recent FreeBSD and aim to be much easier to set up and administer. For example PC-BSD includes easy installation support for the excellent filesystem ZFS which is a bit of work to use in other OSes, including FreeBSD.
Linux has broader driver support and greater overall popularity than FreeBSD, though most agree the actual BSD kernel and networking performance are technically superior. Note that FreeBSD has a Linux compatibility mode which can run Linux applications. See Red Hat as described below for support services, applications software, and more.
Linux is now being installed and pre-configured on servers by IBM, HP and other mainstream PC vendors. More importantly they are also providing technical support for Linux. This should make Linux even more attractive to companies looking for a commercially-supported (but open) operating system.
Ubuntu is a very popular desktop version of Linux.