It's a hard question to answer, since it depends upon where you're riding, how far from civilization/your truck, maintenance abilities, etc..
For me, I ride singletrack that is typically 25 to 35 miles loops. The trails tend to cross many backroads/jeep trails so I could shorten a route in an emergency. Over the years, I have reduced what I carry and now carry the basics or bare minimum (still more than my buddies carry).
I used to use a fanny pack, but I don't like the weight on my hips and banging around. I use a camelback with extra storage capabilities. I bought it at Costco (I think the pack is called High Sierra)... anyways it looks like the camelback Mule version. I carry my water, some granol bars, my tool wrap, tow strap, car keys, cell phone, photocopy of license and insurance info, and $20 (for emergencies).
In my tool wrap, I have the basic necessities to fix most things on the trail. This covers most of the minor repairs and gets me back home.
8, 10, 12, 13, 14, 15mm combo wrenches (these have a different size on each open end). You give up the box end but you minimize the number of wrenches.
T-handle with similar size sockets including 6 and 7 mm (for carb & hose clamps).
Multi-purpose tool (like Leatherman) which has pliers, sidecut, knife, etc.
Folding tool with allen wrenches (4, 5, 6mm) and screwdrivers (straight and phillips). Tusk makes a nice cheap one (from RockyMountain ATV/MC).
Miscellaneous items like zip-ties, master link, and section of fuel hose (to transfer gas in emergencies).
I do not carry tire irons or tube, unless it is a "long distance" trip. Most trail rides, I can ride back with a flat tire, zip-tied to the rim in emergencies. So, I typically don't carry them.
I carry what Matt does in a fanny pack, plus a couple of plugs, mini chain breaker/links/master link, wire, zipties, misc hardware, and some quicksteel (the mini chain breaker made a nice brake pedal tip on my bro's Husky 450). The Camelback is a great place for the fuel hose, you can eaisly coil up a few feet. Large electrician size zip ties for fixing a flat to the rim too. For longer expedition type rides in new territory we carry a couple tubes, pump, tire irons, shifter, and throttle cable amoung the group.
If your looking for a ready to go pack, Cruz Tools offers one. I purchased the Roadtech tool roll for my wife to carry in her fanny pack. I'm impressed that it has a good assortment of decent tools and I steal it for my street bike trips.
Nick's got a point. I always drag tools around and have yet to use 'em.
I'm curious why folks opt for a fanny or backpack over a fender bag.
I got a little (MSR?) bag that was supposed to hang off a roost guard with zip ties and instead zip tied it to my fender and added a velcro strap to suck it all together so tools don't rattle around. It holds more that enough stuff and I never notice it.
My bike is always among the best prepped on any given ride. Usually my tools are used on other bikes to keep things going. Also, you do not want to be caught in the woods here with a dead bike, could make a bad day worse.
Fender bags tend to beat themselves apart from the inside out. Think about it, the rear of the bike is moving a lot more than your butt.