I've got a big ass camelbak, but I don't really go too crazy with carrying lots of junk unless it's an epic ride, or there's a chance I could be stranded in the wilderness in unforgiving conditions overnight.
I carry:
A waterproof windbreaker
An awesome Crank Brothers multitool
Tire levers
A CO2 inflator and a backup mini-pump
Usually some energy gells
My phone depending on which trails I'm riding
A tube and patch kit
5 bucks that I leave in the Camelbak in case I have to buy a tube from somebody on the trail, or just need some emergency cash.