thttpd is a simple, small, portable, fast, and secure HTTP server.

Simple: It handles only the minimum necessary to implement HTTP/1.1.

Small: See the size comparison chart at
http://www.acme.com/software/thttpd/notes.html#sizes.  It also has a
very small run-time size, since it does not fork and is very careful about
memory allocation.

Portable: It compiles cleanly on SunOS 4.1.x, Solaris 2.x, BSD/OS 2.x,
Linux 1.2.x, and OSF/1 (on a 64-bit Alpha).

Fast: In typical use it's about as fast as the best full-featured servers
(Apache, NCSA, Netscape).  Under extreme load it's much faster.

Secure: It goes to great lengths to protect the web server machine
against attacks and breakins from other sites.

thttpd also provides URL-traffic-based throttling and virtual hosting.