A guy who was local to me, when I was a kid, wrote multi-user BBS system (called "MUBBS" originally-- I don't remember what the name was changed to later) in Turbo Pascal that had a preemptive multitasking loop running in x86 real mode to handle multiple lines simultaneously. The coolest part was the console was just a "line" so you could logon to the board and interact while somebody was online with the BBS, too. Most other DOS BBS packages were only available for the SYSOP or the caller individually.

Edit: Ugh... I'm gonna have to go back to floppy images to find it. There's a "MUBBS" for Mac from 1992 showing up in search engine results but that's not the one I'm thinking of. It was more like 1989 or 1990.