I have to qualify my answer a bit. I'm a back-of the pack runner, so I have to plan to a) be out there a LONG time, and b) figure that the aid stations will run out of stuff before I get there. I've run lots of races where the aid stations were dismantled before I arrive. I try to be dependent for the aid stations for nothing besides water, and even then as I've described I try to carry a little water bottle along for emergencies. I read the race website carefully to know what to expect, but I also don't rely on race management to keep their promises. Better happily surprised than sorely disappointed.
That said, any added weight slows a person down when running. So I have to strike a compromise between added weight and being prepared for anything.
For a full marathon, I wear:
* A bandana around my wrist or tied on my waist strap. This is mostly for wiping sweat or washing off salt, but I'm also prepared to abandon it in a portapotty should that become necessary.
* A gel flask filled with my home brew clipped on my belt, or I carry 5 sports gel packs (for every 4 miles).
* Sunglasses and a visor to reduce eye strain/fatigue
* A waist pack with my essential supplies (everything that will get ruined by sweat is inside plastic):
- A pace chart covered with clear tape and safety-pinned on the outside
- 1 car key - safety-pinned in so I don't drop it
- $20 in a ziplock - safety-pinned in so I don't drop it (plus a subway pass, etc., depending on the race location)
- 10-12 salt capsules inside a plastic canister (a mini M&Ms container)
- A copy of the race map inside a ziplock (for emergencies or meltdowns)
- A few hard candies and/or glucose tablets to fight off the wall at bad stretches of the race
- Chapstick (can double for sunblock or temporary lube in emergencies)
- Eyedrops (I wear contacts and when you're dehydrated they can get bad)
- A few emergency drugs (NOT more than the maximum I think I need, because I tend to lose track of what I've taken when my thinking gets foggy late in the race), including:
- two 8-hour Tylenol
- two ibuprofen
- two Rolaids or Tums or a Pepcid AC
- two Benadryls or other allergy drug in summer
Lots of people have other strategies, and if I were faster I wouldn't be on the course so long and wouldn't need so much stuff, but for marathons I'm equipped more or less as a self-supporting ultrarunner. I've found out through long trial and error that this works for me!
Perhaps I should add: I almost always go to races alone - which means I don't have someone to meet me at the finish line, or to take my warmups, or hand me some Gatorade at the halfway point, or carry the car keys for me, or that I can phone to bail me out if I have problems. Everything I carry is something that I've really, really needed at one time or another that has turned out to be a life-saver on a race course. I may be able to pare it down a bit if I get faster and/or I'm at a race where I know that I can count on the aid stations being well-stocked.