HGttG is grand.
Zork and nethack hold special places in my heart as well though. [But yeah, tiled nethack is fun in that special everything you see /will/ kill you instantaneously way].
However in terms of things i play frequently, there are the muds of addictiom +3.
The foremost one is the Realms of Despair that i've been on for the last several years fairly consistently [though a lot less so during school terms]. Wonderful playerbase averaging 300 at any given time, the economy's a bit wonky in some regards, but that seems to be a problem with virtually any online game [*cough* SoJ's].
It's been partly catharsis and a way to avoid keeping logging back into my /previous/ mud of addiction +7 which was Achaea for a couple years before that.
Main website is
http://realmsofdespair.com/ though there are lots of really exceptional user sites as well. If anyone ever feels like logging on, i run Acherontia and Hikedha, and would be happy to show you around sometime if i'm on.
My other current favourite is a smashing AvP casual lightning pk mud that's been around a few years, which i've followed since before it was even really in proper development. It's way fast-paced, but totally laidback and fun. Unlike most pk muds, almost everyone plays it like it's just all in good fun, you don't expect to live beyond a minute or so after deploy generally and there's not really any penalty for dying aside from sometimes scrapping eq [which is all bought cheaply in your race hq, and is all basically uniform, with variation primarily on weapon preference]. Unfortunately over the last several months there has almost never actually been people on, although before that there were usually at least half a dozen active players at any given time. A few days ago though i logged in to check and had a whole dozen so it may be on the upswing again. Drop in and check it out, the code and design is way sleek. I run a pred named Khnemu, xeno named Qhnemu and a 'rine named Lemmus
http://finalstand.archsysinc.com/avp/
[actually i should caveat that with the fact that a few days ago the server seems to have disappeared via telnet, so i'm not sure what the status is actually]
Also, I can kill you with my brain.