The original QSpy protocol, which then became the GameSpy protocol, and later still made way for Valve's Master Server protocol.
I'm not sure what other protocols exist today for tracking a list of servers, providing information on them, and are as up to date as the frequency of the heartbeats from those servers, but I suspect there are similar protocols out there, and I'm just not familiar with them.
Unfortunately despite how elegant the QSpy protocol is, most modern video games no longer provide server browsers as first-class features, eschewing them in favor of matchmaking services or publisher provided dedicated servers.
I'm not sure what other protocols exist today for tracking a list of servers, providing information on them, and are as up to date as the frequency of the heartbeats from those servers, but I suspect there are similar protocols out there, and I'm just not familiar with them.
Unfortunately despite how elegant the QSpy protocol is, most modern video games no longer provide server browsers as first-class features, eschewing them in favor of matchmaking services or publisher provided dedicated servers.