Backpack / Messenger Bag / ???

[mushinspace]

So, even with the saddle bags, I am pretty much storage constrained ... 1 side of the saddle bags is always full with just my rain suit, which only leaves 1 bag left.

What do most people use to haul "stuff" around? I need to find a solution for bringing my laptop and stuff with me, as well as more generic "stuff" on occasion (like small grocery trips, etc).

I thought about a luggage rack, but that's a pretty small rack. :(

Here's a bag that I was looking at online from the bike (non-motored) world:

http://www.chromebags.com/products/bags/show/42/

But it's not all that general purpose. What I *do* like about it is the "seat belt" latch that lets you strap the bag on w/o having to throw a loop over your head (helmet) and arms/shoulders (bulky jacket).

Anyone got any other suggestions?

Thanks!
Sam