Lid & puncture fixing stuff. (The powerlink's in the puncture outfit box as well)
Oh, & me Alien.

No single item but I usually use at least one of these…

Allen key set, chain tool, zip ties, 3 inner tubes, pump, brake pads, power link, tyre levers.
2 or 3 water bottles and usually a thermos with coffee.
Paclite jacket and except if its baking hot, overtrousers
Spare contact lenses and glasses. Bit of money. Phone.

Helmet (goes without saying).
Map and GPS if I'm on my own.

Helmet!

Saddle Bag: Alien II, spare Hope Vision 2 battery, innertube, tyre levers, puncture kit, powerlinks.

Pump in bag/rear pocket.

A paperclip

Not really sure why, but I've asked on more than one occasion "Have you got something like a paperclip?"

After an article in SingleTrack (i think) I've been riding with minimal kit. Granted only local jaunts, so the walk home wouldn't be more than a handfull of miles.

I can't remember the last time I had a puncture, and never (touching wood) had a mechanical that stopped it rolling.

Just me, the bike, helmet and clothes. Liberating. Though I guess without the clothes would be more liberating!

Multitool tube and pump. patches and glue, power links x3 and a couple of links of chain. M6 Nut and bolt. M5 nut and bolt Unless is just a mile or two around town when I carry nothing.

since getting an alfine its a 15mm spanner and some long nose pliers to add to spare tubes, patches, levers, some spare chain links, mini pump, multi tool, mobile, mini first aid kit, energy gels and a fiver

4+5mm allen keys, cash card…. oh, and a copy of the Road Traffic lawbook, to show MTBers driving to the trail centre. 😉

Asthma inhaler. Everything else is a good idea, but not life or death.

EDIT : apart from a helmet.