Apache

Apache is an (some would say the) open source webserver; used more than any other webserver in the world. It is available for easy installation from FreeBSD's ports tree in several different flavors, including pre-configured versions with and without OpenSSL, mod_php, frontpage extensions, and lots more.

For most purposes, Apache is absolutely the only webserver worth considering. In very high-volume (or tiny-hardware) contexts that need absolute maximum hardware efficiency at the expense of flexibility and configurability, however, you may wish to consider thttpd, which is designed for absolute bare-bones simplicity for the delivery of static (no CGI, no SSI, no PHP, no nothing) HTML.