Currently there are 3 truly online roguelike games:
– PernMAngband it was actually TomeNET, which was renamed cause copyright Pern issues.
– TomeNET is the most popular online roguelike; but also there are two more online roguelike games – MAngband (was 1st online roguelike ever) and PWMAngband (got some pretty revolutionary stuff). I myself develop Tangaria (Tangaria).
– I tried hard to be able to play Nethack online, but didn’t get success; it seems all devs abandoned their work long time ago.
Also some online games with strong roguelike elements:
2) Mystera Legacy
3) RPG MO
4) RotMG (not too roguelikish, but interesting)