Multinet is a curses-based solution to sharing a dial-up Internet connection using IP masquerading. It keeps track of time spent online, billing information for each user, and user manangement (so that users cannot kill each others connection).
You can download the source by selecting the sourceforge link below. Multinet has been tested under Linux (Redhat Sparc) but pretty much any OS with a C compiler, curses (or ncurses) library and similar serial device handling should be ok.
(c)Dominic Tristram 1999 See my home page, if you want