Once you've decide on a key carry method and a flashlight, I think there is one more essential tool you need to add--a one piece multitool (OPMT)(I don't know anyone else but me calls them that, but I don't feel like writing "one piece multitool" over and over again). Yes, they can be ridiculously expensive and hard to find. Yes, you will use it as a beer bottle opener 94% of the time. Yes, they all kind of look the same. But, in a pinch these prytool-bottle opener-screwdriver-wrench-box punch-butter knife tools are really, really helpful. If you are opting for a SAK/Leatherman you can skip a OPMT, but having both can be helpful as no SAK/Leatherman can work as well as these tools can in prying tasks. Ideally, if space, weight, and places you go allow, you can have both. I can't carry a SAK/Leatherman so I have opted for a OPMT. And here is the tool that started the OPMT craze in earnest, the Atwood Prybaby:
Before I go through the benefits and drawbacks of various designs here are some the premiere makers:
Pocket Tool X
Ewotah Edge and Tools (EDIT: forgot this one)
TT PockeTTool (new maker)
and finally, the evil villain of the bunch, the mega corp:
There are a whole lot of other makers out there that have done one or two pieces, but these guys, they have made stuff for a long time and have many different designs. There are also a few random tools, older designs not purpose-built for the keychain, like:
The County Comm Widgy Bar
The Lee Valley Pocket Wrench
All of these have a few basic functions: prying, screw driving, and bottle opening. Some can open cans, others have exacto knives, and still others have wrench features. I have owned an Atwood Atwrench, a JDR Baby Pacu 2.5, a Gerber Artifact, and now a Gerber Shard. Instead of breaking down every design out there I am going to hit the high points: the necessary tool selection and price.
94%-99% of the time the OPMT is going to be a bottle opener, in all likelihood a beer bottle opener. If the tool is missing this feature, move on--its not worth owning. After the bottle opener, a driver of some sort is probably the next best feature. Every once in a while you will need to tighten a screw on a draw pull or a door knob and these work well for simply tidying up tasks. The most commonly used screw is a Phillips head so your tool should have one of these. Unfortunately, absent a loose bit (which both Atwood and Pocket Tool X have), only one of the makers above can cut a Phillips head into their OPMT and that is the evil corporation Gerber. Joshua Rice told me that it had to do with the need for a water jet cutter, something that is out of the price range of most custom tool makers. As a result, the second best feature is hard to find. I think the loose bit designs could work, but it is another thing for you to lose. A Flat head bit is next most useful and all of the tools on the market allow for a pry end to double as a flat head driver. Nothing special. The tool will, on very rare instances, be used as a pry. Because all of these tools are small you can't generate the leverage you would be able to get in a larger pry. Many have bends in the tool to help generate more leverage and some have v-notches for nail pulling. The pry is only moderately useful because of the size of the tool. Some offer wrench abilities and they stink. They stink for two reasons: 1) small tool equals small torque; and 2) they can't be held at an angle or off the work surface like a regular wrench or ratcheting socket, so you can't get a lot of grip on them. Some tools have other features--a cutting edge (which presents a whole lot of problems for a tool designed to go in your pocket), a ruler, a can opener, and a few other weird options like an oxygen wrench. Of these odds and ends only the can opener seems useful and even then, only to a certain degree. The cutting edge adds so much bulk, requiring either a folding mechanism like the Artifact or a sheath like the Son of Pry Thing, seen here:
In the end, the bottle opener, the screw driving features, and the pry are the things to focus on. An aggressively angled or circular bottle opener works better than a 90 degree one, like those found on Atwood designs. I like the openers on JDR tools better and the Gerber hooks are passable.
Price and Collectibility
As purely tools, vritually all OPMTs are overpriced (Gerber being the obvious exceptions). From a collector's standpoint, these limited run, handmade pieces are like honey to bears, especially among EDC fans and gear geeks. But as tools, there is no reason why these tools need to be so expensive. Peter's designs are beautiful. His grind lines are immaculate. JDR tools are similarly nice. But folks these are just pieces of steel with cut outs around the edge. I cannot understand why a tool as simple as an Atwrench is the same price or more, new, as something as complex and useful as a Leatherman PS4. I know one is custom work, but again, it is just a slab of steel. Viewed purely as a tool, this makes no sense. The other problem with Atwood stuff is availability. Like Busse, Swamprat, and Scrapyard knives, they are not really available at the prices listed. In reality you have almost no chance of getting one of the 10-20 pieces offered off his site at the original price. The secondary market is the only reliable way to get an Atwood piece and there the prices are 10-20% more for common items and 100% more for rarer pieces like the one above. JDRs prices are better and his pieces are readily available. He also uses more exotic touches in his basic tools, offering glowing dots and mosaic dots along with more complex edge grinds. Atwood tends to reserve these for his more exotic or limited run items. Pocket Tool X's prices are more in line with what these tools can do. And Gerber's prices are very reasonable. Their stuff, especially the Artifact, is clunkier, but it is also about 10% of the price of other OPMTs.
In the end, I dropped the Atwrench, not because it wasn't a good tool--it was, but because it was heavy and expensive. I did not appreciate it for what Atwood's stuff really is--collector's pieces. It was like being a power tool guy and inheriting a Lie Nielsen smoothing plane--it was too valuable to use. So I traded it. I also had a JDR tool and got rid of it for similar reasons. Finally, while traveling in the back woods of New Hampshire I stopped at a locally owned gas station and right there next to the night crawlers and chicken salad sandwiches (which look like they were made from nightcrawlers) I found a $6 Gerber Shard. It had a Phillips head driver, which my other two tools did not and so I swapped it in. I have been very happy ever since and I sold the JDR for enough money to pay for half of a Spyderco Leafstorm.
You can go crazy with OPMTs these days. I recommend you try a Shard or a Pocketwrench or a Widgy Bar and see how much you use it and how much you like it. If you really enjoy it and would appreciate a more collectible piece go hog wild and plunk down half a grand on a Talonite Mini Son of Pry Thing on ebay. It will be beautiful and probably appreciate in value, but it won't do anything better than what a Shard does (except for cutting, obviously).