Quoted from Wikipedia:
"Starting in the summer of 2008, there have been Chex Quest cooperative and deathmatch games hosted on the Doom Multiplayer Client Skulltag by ChexCommander. Games were hosted daily from July to August, and were popular among many Chex Quest fans. This has proven that Chex Quest, along with many of its GZDoom mods, is able to be played online between players across the world. More information is available on the Chex Quest Fan Forums."