The best old skool dos games were the quest games from Sierra: King's Quest, Space Quest, Police Quest, Quest for Glory, and all of their sequels.
heres a compendium of most of the games that were available for DOS.
a good trip down memory lane.
its all shareware, freeware, and abandonware, so its all legal.
Hey, what's the name of the game the kid plays in 'Big'? Did it really exist?