Here's my commuter. A 2001 Schwinn Mesa GS that I've upgraded with XT front and rear deraillers, LX cranks, XT shifters, cane creek levers, race face seat post, and a garage sale Selle San Marco seat ($2).
About the only thing left from original is the handlebars and brakes. Last upgrade was a set of wheels and an XT cassette. I don't go off road as much as I'd like so I went to some discount store slicks. I had some thinner ones I liked better, but at 6.99 for two, I couldn't pass these up.