never wheel alone, you may be walking home.
my addition to bds5's list;
means to repair/fill a flat, or a thick walled innertube (oba, co2, hand pump, cig lighter pump, ect...)
basic hand tools
small sledge hammer, like one of those 4# ones. i have used mine for just anout anything.
mapp/propane torch, built a campfire for light/heat once while highcentered and drunk at 3am alone.
recovery gear of your choosing
bow saw/small chainsaw.
some of the items would be totally useless for some trips and most of your gear would depend vastly on the intentions of the adventure. however i think most would agree that means of recovery is necessary.