DOOMRL - Doom, The Roguelike.
http://doom.chaosforge.org/ Excellent in every aspect.
ADOM - Ancient Domains Of Mystery.
http://www.adom.de/ May be the reason you won't leave home this summer.
The KROZ Series -
http://members.chello.at/theodor.lauppert/games/kroz.htm - hard puzzle games by Scott Miller, first release from cult Apogee! Download whole series here: [url=ftp://ftp.3drealms.com/freeware/krozfree.zip]ftp://ftp.3drealms.com/freeware/krozfree.zip[/url]
Caves of Thor -
http://en.wikipedia.org/wiki/List_of_minor_Apogee_Software_video_games#The_Thor_Trilogy - another early game from Apogee Software, this time by Todd Replogle. Arcade action game, sort-of Zelda'ish piece. Download here: [url=ftp://ftp.3drealms.com/freeware/thor.zip]ftp://ftp.3drealms.com/freeware/thor.zip[/url]
The Island of Danger -
http://www.hotud.org/component/content/article/43-action/21945 - fast shooting action!
Pyro 2 -
http://www.hotud.org/component/content/article/43-action/20550 - fast, exploding game of action/puzzle genre!
And let's not forget about the wonderful worlds of
and [url=http://vault.digitalmzx.net/]MZX games.