I'm seconding the recommendation for a Surly Cross-Check. It's technically a cyclocross but it can do oh so much more. Gears, single speed, fixed gear, internal hub gears, skinny tires, FAT (700x45) tires, it can be used like a "pseudo" mountain bike with the FAT knobby 29'er (700x45) tires, it can be used for road riding if you slap on some lightweight rims with 700x23 tires. You can also ride like those bike messenger dudes by switching to a fixed wheel, which the Cross-Check can do with way less problems than a conventional frame
And on top of all this, the Cross-Check has braze-ons for fenders and a rear rack, so you can turn it into a good commuter bike too!