Usually I just rock a little camelbak w/o bladder. It holds all my tools, spare tubes, and pump. Sometimes it even holds a change of clothes. Sure my back gets sweaty. But...my everything is usually sweat-drenched after a few miles in the NC summer.

Days when I need to haul a bunch of stuff, I use my huge chrome bag. It holds the camelbak bag mentioned above, my laptop for work, and even a couple tall boys. I find both of these bags comfortable.