AOLserver

To quote aolserver.com directly:

AOLserver, America Online's own web server, is the backbone of
the largest and busiest production environments in the world.
AOLserver is a multithreaded, Tcl-enabled web server used for
large scale, dynamic web sites that sports a long list of features.
AOLserver powers America Online properties such as AOL.COM, Digital
City, AOLMail, AOL Hometown, Helping.org, AOL Search, and much,
much more! Yay!

AOLserver is distributed under the AOLserver Public License. The official site is at http://aolserver.com/.