Windows

IWeb Mini Web Server
Small-scale HTTP for live testing of HTML pages on Intranets. Free download.
http://www.ashleybrown.co.uk/iweb/

Sambar Server
Multi-threaded combined Web, Mail and FTP server.
http://www.sambar.com

Savant Web Server
Freeware download, news, documentation, support pages and screenshots for this open source tool.
http://savant.sourceforge.net/

SourceForge.net: Uniform Server
Free minimal (2 Mb) server that includes Perl, Apache 2, PHP4, MySQL3, and phpMyAdmin. [Windows 95/98/2000/NT]
http://sourceforge.net/projects/miniserver/