I'm a "be prepared" type, and in my wedge I cram a multi-tool, tire levers, 1 spare tube, patch-kit, about 1 ft of duct tape (flat ribbon in a ziplock bag), a few individually packaged wet-wipes (good for toilet paper, bloody noses, first aid, etc.), and 2-3 gu-gels. In my jersey, I also carry a polyesther or gore-tex shell jacket (depends on how warm/cold), some real food bites in easy-to-get-at wrapping (few fig newtons, 1/2 power or cliff bars, etc.), ID and medical insurance information, some cash, prescription sunglasses, and maybe my cell phone. If my wife can't make it to the end-of-the-event, I also carry my car keys. On my bike I carry 2 24-ounce water bottles and a mini-pump. I keep one bottle full of water, and the other empty unless it is hot, and then I add either water or gatoraid to it as conditions require.
I know, I know...where is the kitchen sink, you ask? That all fits into my medium-sized wedge bag and my jersey with no problem at all. I've actually needed all of those things at one time or another on my solo training rides over 60 miles (gets rural fast away from town here), and it became a habit. In big event rides with 1'000s of riders, the duct tape almost always comes in handy with some other cyclist that has broken down on the side of the road too. It weighs nothing, slides into the bottom or the side of your wedge, and can be used for anything from emergency cable/saddle/bar-tape repair, first-aid bandage adhesive, to serving as a temporary internal tire patch in the event of a large hole or side-wall tear.
I carry more than most of the road/racer riders, and a lot less than most of the touring and ultra-distance cyclists around here.