www.boa.org/
Boa is a single-tasking HTTP server. That means that unlike traditional web servers, it does not fork for each incoming connection, nor does it fork many copies of itself to handle multiple connections. It internally multiplexes all of the ongoing HTTP connections, and forks only for CGI programs (which must be separate processes), automatic directory generation, and automatic file gunzipping. Preliminary tests show Boa is capable of handling several thousand hits per second on a 300 MHz Pentium and dozens of hits per second on a lowly 20 MHz 386/SX.