One of the oldest file sharing programs on the Net, Hotline is still in use today, and I recently had the opportunity to confirm that. Even though the parent company (who originally intended to sell the product as a BBS/collaboration tool) seems to have gone under, there has been a resurgence in its use that
How to spot if your users are running it:
Hotline uses (at least) TCP 5500, but recent versions (official 1.8.x and some third-party clones) have implemented both SOCKS and HTTP tunnelling support, so it is harder to spot and block (but you can configure Squid to only proxy "normal" HTTP requests).
Hotline was traditionally a single-sourced application, but there have been recent efforts in cloning the protocol (fortunately dampened by the emergence of other file-sharing tools). A simple Google search revealed the following: