Dungeon Master clones are quite freely available...
CSB http://dmweb.free.fr/?q=node/851
RTC http://www.ragingmole.com/RTC/
...and those early Might & Magics are sitting in my backlog.

But how the hell do you map then without using pen and paper? Is there a utility designed specifically to draw rooms on a grid, and mark them with notes and comments?
There are tools like that for interactive fiction (like Trizbort), but searching for RPG-specific stuff brings up mainly junk for drawing your own campaign maps.

MS Excel (or Google Docs or Libre Office). Gives you all the benefits of graph paper but you also can leave very long notes and easily color-code stuff.

I used to go by memory... and if the level was too big, dropping some useless loot to mark the road was a big help.

I always loved drawing my own maps on squared (checkered?) paper.

Same here. In fact, I use quadrille ruled graph paper for most everything I need to write. Must be my inner engineer.

Ah-hah! A thread on the subjects
http://www.rpgcodex.net/forums/index.php?threads/any-good-dungeon-mapping-software.65873/

And I've struck gold - Dungeon Mapper!
http://www.zerker.ca/zzone/2010/01/10/dungeon-mapper-0-1-alpha/
Tile-based, simple, keyboard-only interface. Walls, doors, switches, inns - all the basics are there.

I did the same thing. I was playing Might and Magic using OpenOffice Calc as a mapper. but it sucked so hard Alt-Entering in and out that I just scrapped it and bought myself a squared-paper pad.

Gets used for everything from Dungeon Master to Grimrock now.

Like this?
http://www.angelfire.com/ny5/telengard/graph.htm

Haha, no. I never tried mapping anything of that size. If that's the kind of dungeon you need to map, my guess is someone on the internet has already done it for you, if you look hard enough.