There is some very good advice above. I recently bought a very inexpensive (and light) multitool from Performance that has screwdriver tips and an assortment of hex wrenches that fold out. I had been carrying a lot of different sized L shaped hex wrenches and a hex bit set. This is an improvement in weight and not being able to lose items. I found I still needed to carry a couple of the larger ones, and an extra 5MM for the saddle clamp bolt (one on each side). I will check out the previously mentioned Park tool to see if it has a better mix of sizes.
General advice on tool "kits" - get your tools out and go over your entire bike seeing what fits where - then leave the tools that don't fit anything at home to save weight. Make sure you have something that fits everything and that you know when and how to use it. (Obviously special headset, bottom bracket and crank-puller tools may be optional because they can be heavy - just make sure these things are correct before you set out because any of these that loosen up many miles from a well-equipped bike shop can do a lot of damage.)
A very lightweight and important addition to your kit starts by making sure that you have a Sedis chain on your bike - because it has a master link that doesn't require a chaintool. If you are going long distance - carry the extra links that came off your new chain (you did put a new chain on for the trip didn't you?) a chain tool and a couple of extra Sedis master links. With this you should be able to repair anything that goes wrong with the chain (including shortening it to limp along length if your rear derailleur totally falls apart). Practice with the chain tool on your old chain - they require some experience. Considering how important the chain is - you want to make sure you are prepared to clean and lubricate it regularly on long trips.
Someone mentioned carrying a tire "boot" and an extra tire. I second this as some tire damage (like loose brake pads cutting though the sidewall) is terminal. I also have a thumb tack with a plastic head on it in my tire repair kit - this is handy for probing the tire for glass and thorns - so you don't patch the tube only to have another flat five miles later. A pill vial with baby powder in it would be handy to powder your new tubes and repair patches on the road - but I admit I've never bothered to carry one on tour.
I have a tiny adjustable plier in my kit that weighs almost nothing and is very handy for grabbing brake cables or the other side of a clamp bolt when the wrench that fits is on the other side. I also have a tiny crescent wrench that I cut the center of the handle out and filed down to the minimum weight. Over the years these have stood up well and been used a lot. Neither of these is over four inches long and they are quality tools - not cheapos.
The most effective and lightest item for your tool kit is preventative maintenance before you set out.
