I have 2 Timbuk2 messenger bags. A medium and a small. The medium can fit almost everything and the kitchen sink. I use that primarily for my commute to school/work. The small fits my laptop nicely with some papers and other odds and eds. In the medium I carry on a typical day:
A couple spiral notebooks, papers
U lock and cable
At least 1 or 2 textbooks
Frame pump
Tube
Tire levers
Deodorant 
Mac iBook
They're great bags, highly waterproof. Durable. But not exactly cheap either. But I had to get the custom colors
"Only the meek get pinched, the bold survive"