At the last minute, I ordered a waistpack with a pouch and two water bottles - basically, a glorified fanny pack. It worked, but since then I've experimented with more options.
This isn't the originally one I had (it came in sizes, rather than being fully adjustable, and after a while it got too big! I decided to buy a different one that could be more versatile). This is from the Nathan "build your own built" series. It has two 8-ounce water bottles (and a pouch that Abe misplaced somewhere during marathon training). Abe used this for the marathon, filling one with water and the other with a homemade energy gel made with molasses and honey.
Pros: Versatile - even the belts with fixed components offer a variety of bottles and pouches to suit your needs. No weight on your shoulders or in your hands.
Cons: I've found it hard to find one with a good fit, that has enough stretch to be comfortable, but tight enough to not bounce around while you're moving. It really is just a glorified fanny pack.
I've had the Amphipod 12 ounce insulated bottle for a while; and bought the Nathan 10 ounce bottle the day before the marathon. The latter is nice because it has a thumbhole; on the Amphipod bottle your thumb is just kind of hanging out on top.
Pros: No bouncing around on your waist or back. Less weight if you don't need to carry as much for a less-long run. The cheapest - as low as $15 or $20.
Cons: Having it on your hand can be annoying after a while. Less capacity. The weight of the holder makes it a bit deceptive to judge how much water is actually left.
Pros: Lots of pockets. Huge water capacity.
Cons: More expensive ($100+, but after trying it on at the REI store, I found last year's model online for less). Difficult to gauge how much water is left during a run. Even though there is mesh that is supposed to be breathable, it does create more sweat on your back in the heat.