Under another topic, I mentioned that I use inexpensive tool bags to carry my stuff.
I've got quite a few, but this is the one I tend to use the most. It's an off-brand that I bought for @ $8 USD. It's about 12wx9hx8d in inches, closed.
This is what it looks like chock full:
Here's a look inside (in "kitchen sink" mode :-))
As you can see, it can carry a lot -- here we have the body, and lenses to cover every FL from 10mm FE to 300mm, and then a jump to 510mm if I really need the reach. The AF 540 FGZ is under the 50-135 on the body. I usually don't carry this much stuff -- as loaded, it's about 12 lbs.
It's only padded on the bottom, and the using the separate neoprene lens bags is not really fast, but the lenses are pretty well protected. I always take my time changing lenses (usually sitting down to do it), so it's not really a handicap for me.
The "stealth" aspect (as Ira put it) is important, because I really don't want someone looking into my car and seeing a bag that shouts "Expensive Camera Equipment" sitting on the passenger seat. I always have a cheap jacket in the car, and will casually drape it partially over the bag if I've got to leave it for any length of time.
I also carry a cheap styrofoam cooler that the bag will fit into in the trunk. If it's really cold or hot out, and I need to leave the camera in the car for any length of time, I just drop it into the cooler, and the camera and lenses hardly change temp, even after a few hours.
I don't necessarily recommend this type of bag, but it works for me, and I think it's good to show Out-of-the-Box methods
-- Well maybe not quite as OOTB as 2many's. . .:-) that was really good. . .