NaviServer is based on AOLserver (version 4.10), AOL's open sourceweb server. The NaviServer project started as a fork of the AOLserver project. It is different in that it wants to be more experimental and aggressive with new feature development.
Historically NaviServer was the original name of the server, a closed-source product by a company called NaviSoft in the early 90's. It was bought by AOL in 1995, and released as open-source in 1999 after they released Mozilla. This friendly-fork takes the code back to its original name.