WinPopup was great. We had a similar program on the Windows 3.11 systems using Banyan VINES in the mid-90s that could, uh... send a message to the entire DoD network* if an enterprising person specified "" as the recipient. Bonus: recipients didn't need to be running any specific program to receive the sudden message as an alert box, interrupting whatever they were doing. Doing this on such a large network - which was likely never intended - had bad consequences.
just the branches of the military that happened to be using VINES
just the branches of the military that happened to be using VINES