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.
Apache fits most purposes well, however when configured with PHP and/or mod_perl its memory requirements scale upwards dramatically, so for particularly high-volume applications a separate server for static content becomes very desirable. Apache itself is reasonably light when the aforementioned modules are not loaded, however thttpd is even lighter. A reverse proxy setup also solves the problem well.