it all really depends on what you want it for and what you're going to put in it. I've got the Icon Primer Tank Bag. It's small, I can keep my phone, wallet, keys, bottle of water, and my rain liner for my jacket and a sandwhich. It also has some hidden straps that make it into a back pack... a very small girly looking backpack lol. I picked it up from someone on another forum for $35, i think retail is around $50.
If you're just looking for something small there's also teh Joe Rocket Manta Jr Bag. Small, has a cellphone pocket, so you can look down and see who's calling and if it's worth picking up. You could probably fit a little less than in the Icon Primer bag... (pretty much all the stuff I put above minus a rain liner.
There's the Cortech series, which are bigger and can hold more stuff, plus they've got a clear plastic piece which snaps on top of the bag. Ideally it's meant to hold a map in it, btu I think an EZ Pass could read through that, not 100% sure on that part though. They also make a series that gets pretty big which you could probably put a small child in (slight exaggeration, but they make some that let you store a good weekend worth of clothes).
There are tons of different bags, it just depends what you want in it.
