A lot of good replies above, but... here are some more
Selected items I think are very important from above plus a couple of tips I use.
1. Mini roll of Duct Tape, when they say a 1,000 and one uses they really mean it.
2. Leatherman's tool, another item that has 1,000 uses and you can wear it.
3. Small pen light, I keep a small one on the end of my Jacket zipper tang, you can always find it when you need it and you never ever forget it.
4. Phone number to your insurance and or road side assitance number.
5. Spare key in the wallet, doesn't do any good if it's locked in your bike saddle bag or under the seat.
6. Make sure you have a list of friends / relatives phone numbers in wallet, your cell phone might get wet and be toast.
7. Small zip lock freezer storage bags, a couple of these can be really handy to make sure something stays dry that needs to stay dry like your cell phone.