Every significant release of FreeBSD is available via anonymous FTP from the FreeBSD FTP site:
For the current 3.X-STABLE release, 3.5.1-RELEASE, see the 3.5.1-RELEASE directory.
The current 4-STABLE release, 4.6-RELEASE can be found in the 4.6-RELEASE directory.
4.X snapshots are usually made once a day.
5.0 Snapshot releases are made once a day for the -CURRENT branch, these being of service purely to bleeding-edge testers and developers.
Information about obtaining FreeBSD on CD, DVD, and other media can be found in the Handbook.