3rd in my ongoing "I have no DSL so let's dust off the pile of single player games and see what's worth keeping" BiiF series is Roller Coaster Tycoon 3. I've had a run of really bad luck with sim/building games lately (Spore, City Life, and Civ 4 all looked promising and all disappointed me), so I was starting to wonder if I'd just gotted tired of the genre entirely. Turns out, no. Roller Coaster Tycoon is pretty damn fun.
The title of the game gives you a good idea of what sort of game it's going to be. You build an amusement park and try to make money with it. And for what this game is, it's implemented REALLY well. Your success or failure is determined by a horde of simulated parkgoers' ("Peeps") reactions to the stuff in your park, and you can measure it by looking at high-level stats on how much money each ride is making, or follow one Peep around on his visit to see what he does, or lots of levels in between.