I have a Topeak 18+ for the road bike and a larger Specialized EMT Comp for the mountain bike.

The EMT Comp has all the tools you will ever need on the road (or trail), but it is large and heavy. The Topeak mini 18+ was the smallest lightest thing in the store I went to that had a chain tool. I can probably take apart my road bike (well mostly take it apart anyway) using only the multi tool.

I also try to remember to pack my swiss army knife, as all bike tools lack a blade.

I also carry (in the saddle bag) my phone, a my credit/ATM card, ID, my medical insurance card, a presta valve adapter, nylon tie straps (VERY useful), and wrapped around the seat post mounted tail light I have a small roll of velcro straps.

The mulit-tool lives in its pouch, which is also on the seat post because the saddle bag is full.

The jersey pockets contains a crash pack (borrowed from the jet ski), a spare tube, a tire patch kit, the CO2 inflation thing, a couple of CO2 canisters, and I think that is all.

Probably too much stuff, but I will look cool when someone on a group ride (hopefully a chick and not just some dude) needs duct tape and I have it.