Shareware version of Stars! was quite good. Pity stuff got really interesting at about tech 11/12, and it cut off at level 10 tech. I also learnt a lot about hex editing, because you couldn't make a custom race unless you edited one into the .exe, so that's what I did.
Dungeon Keeper's demo wasn't great, but about two months later there was a level editor released for the full version on the same magazine's coverdisc. It worked on the demo version too (although I'm pretty sure some of the enemy graphics weren't able to be used, they still had functional abilities). So I made many cool levels for free.
Syndicate: American Revolt demo wasn't too bad. Mostly due to the sound, especially of the air strike. I even learned how to rip .voc files from the .exe from that one, just because I wanted the rocket launcher and shotgun sounds in my game so badly. Never finished the game, but learnt a fair bit about how to play .wav files through a PC speaker in TP6.0 (because I was enthralled by the Space Hulk's demo and it's ability to play voice through a speaker, and MegaLoMania's).
Will second Doom and and UT and CtF. Many hours lost on those ones. Got an idea MoM and MoO2 got demos too at one point, which encouraged me to get a full copy of them.
Does anyone here want me to make up a touchscreen interface for Capture the Flag for Magic Dosbox? Might be a good little hobby project for the day. It's pretty mouse driven anyway, but a few extra hotkeys for map scrolling and next player might help (I've done it for Stars!, SC2000,DoomRL 0.8 and Pyro][, so this one would be pretty easy in comparison). Then you could play it on your phone! Yay