Any tycoon game, zoo tycoon, roller coaster tycoon etc
Do you like logistics simulations?
You've probably played already , but otherwise check out OpenTTD and Simultrans, both are quite fantastic and have decently large playerbases. Both also are activly being maintained and developed.
Otherwise in the 90s, Detroit and Airbucks were both pretty fun, but they are very out of date today, and nothing of that type really exists in today's game market. Hopefully someone will fill that void
Is Detroit the one from 1994? http://en.wikipedia.org/wiki/Detroit_(video_game)
There was another one I played not too long ago, didn't have graphics and it was really hard to make any money with. Wish I could think of it.
While looking up Detroit, I came across The Corporate Machine http://www.gamespot.com/the-corporate-machine/platform/pc/ which looks to have reviewed well.
Also, a top 25 Tycoon game list http://www.squidoo.com/the-best-tycoon-games with one called Corporation Inc. that looks like a Sim Tower/Hotel Giant type of game.
Dwarf Fortress would probably fall into this category, but holy god is the learning curve steep...
There's good sports manager games out there for soccer, football, and baseball. Forget their titles though.
Football Manager, http://www.footballmanager.com/
This is really more of a sports simulation but it's so incredibly deep that you can play it like a business/financial sim as well. You can hire coaches to take care of all of the actual on-field strategy and lineups etc and just concentrate on financial aspects like how long to hold star players before you sell them, how often you want to upgrade your stadium, how much to pay your training staff, etc etc.
One of the amazing things is you can play as teams at the top level with hundreds of millions of dollars of revenue or as tiny teams with semi-pro players that operate on a budget of thousands of dollars a year.
Eve is probably the best business game I have played in quite a long time. The bonus is that you are playing against real people and not NPCs.