An Owner's Manual This is the most valuable tool you can own. It'll tell you everything you need to know about your motorcycle. Including how to tell when something's gone wrong.
Tire Pressure Gauge Even a cheap one is better than none. If you can't check and adjust your own tire pressures, you shouldn't be riding without adult supervision. Check them once a week or whenever you ride the thing.
Four-way Screwdriver A halfway decent four-in-one screwdriver with interchangeable tips cost about five bucks at your local big box store. Use it to change that burnt out tail light bulb or tighten that loose light switch in the hall.
Medium Size Adjustable Wrench You can use a good adjustable to do everything from tighten a loose bolt to open a can of beans. Figure on spending about 15 clams for a good one.
Locking Vice Grips Your bike tips over, and breaks a shift lever, how do you get home? Clamp the vice grip on the stub of the broken shifter. Oh yeah they work well as pliers to.
Flashlight Peer into dark spaces, signal on-coming traffic, find out what evil lurks in men's souls. Okay, the last is a stretch, but a good flashlight is worth its weight in just about anything, and should be carried by anyone that ventures out after dark or just wants to see where that drop of oil is coming from.
Shock Adjusting Tool Carrying a passenger or even yourself is a lot more fun when the shock preload is properly adjusted and its lot easier to do when you've got the proper spanner. If your bike didn't come with one, and most don't, order one from your favorite local shop.
Battery Tender If you don't ride your bike on at least a weekly basis, the battery should be kept up to snuff with a plug-in charger. I use one on my infrequently ridden dual sport bike and the battery has lasted nearly 6 years.
A Funnel I have a real aversion to riding my bike when it's low on oil, and an even bigger one to spilling lubricant all over it. You should be checking, and if need be, topping up your oil on a regular basis. Having a funnel handy makes the job a whole lot easier.
Cruz Tools Econo-Kit (or equivalent) Okay this last is something of a ringer. However, if you want to delete items two thru six buy one of the prepackaged tool kits that are on the market, and yeah, I unabashedly recommend one of Cruz–tools kits. They contain everything you need to perform minor (or not so minor repairs) and start at about 40 bucks.