Nuke's Pick for C64 Sysops!
Color 64 is a computer BBS system that was very popular for the Commodore 64 during the 1980s. It was written by Greg Pfountz. Color64 was advanced for its time. The I/O was all written in machine language.

It was also composed in modules, each written in BASIC. Each module would be loaded into memory as a user went from one part of the system to another. For example, the login module was what handled the answering of the phone and the verification of login info. It would then load the main menu module. Going into the message boards would load the message board module. Online games worked the same way.

Recommended hardware (or emulation thereof): 512K RAM MEU
In testing, could not get this to reliably work with C64 Wifi Modem (yet....)