Get a good multi Tool such as Park's MTB3; you don't need to carry tire irons while MTB riding with this tool-BUT you will with a road bike because the tire irons are too wide for road tires. I have the original version of this tool and it works great, in fact I turned to it when at home dinkin with the bike because it's easier to get to since it's already on the bike.
You need a patch kit; I prefer the glueless because they work and their faster to use with no dry glue tube surprises. By the way, glueless patches ARE NOT temporary patches as some argue; I have a 3 year old tube in a road tire with 2 glueless patches and their still working, I have a MTB tube with 5 and still holding air.
Also carry a boot patch in case of a large tire hole.
I would recommend carry a spare tire-BUT on a MTB that's impractical due to the size of MTB tires. I carry a spare for my road bike but it folds up into the seatbag, on my mtb I can't do that. But I actually ride in more remote places and further from home on the road bike then I do on my mtb so a spare tire is more important on the road.
But carry a spare tube.
Get a cheap folding pair of pliers Leatherman type of tool; I got mine from Target for $12.
Of course don't forget a pump! I use mini for both road and mtb, but with a mtb a mini is easier to use because the tire pressures are lower.
Zip ties I'm not sure if I would bother, but you could carry a small roll of duct tape.
Some people I know carry a fiber spoke or two in case a spoke breaks but I never had a problem with that so I don't.
Also some people carry a spare chain link, but again I don't since I never broke a chain. Also if you chain did break you can reassemble the chain with one less link which means you may not be able to use your largest or smallest gear but the middle gears will work just fine.
My last 3 suggestions are probably just a waste of time and bag space, but only you can decide that based on experiences you might incur.
Last edited by froze; 05-28-05 at 02:34 AM.