I carry stuff to fix a flat, tire boots, a pocket knife, and this awesome set from Topeak (easily my most used tool - it is perfect for installing water bottle cages, amongst other things)
TopeakŪ Cycling Accessories ? Products - Ratchet Rocket? Lite, 10 bits, zip ties and a roll of electrical tape.
If I have something more major happen that I can't easily fix on the road, I'll just call someone to pick me up.