I carry two multitools with me, as well as a couple of items on my keychain.
One multi-tool is the SOG S60-N PowerLock. It's a bit bulky, but it's capable of pretty much anything I might need--knife, pliers, wire cutter and stripper, Phillips and flat head screwdrivers, can opener--and then a few things I don't need much at all. The pliers are sturdy and comfortable to use, and all the blades lock, which is important. It also has a good feel as a screwdriver, with the head just slightly off-center. The one thing that does feel a bit off is the knife, since the blade faces towards the inside, rather than the edge, when it's open. Still, it's long enough that the handle rarely gets in the way. This tool is very useful when you need it (just in case you have to defuse a bomb or something), and not a pain to carry around when you don't.
The other multi-tool is my iPhone 3G. Don't laugh! Aside from the standard phone and iPod functionality, and the other things which you expect from a modern phone (e-mail, camera, alarm clock, calendar with reminders), it also has a GPS integrated with Google maps, which I use a lot more than I would have expected before I got it. The GPS isn't always very accurate (it can be off by a couple of blocks sometimes), but it does land you in the general vicinity well enough that you can find your way. You can even browse the web better than with most cell phones, though in this aspect you'll likely find yourself wishing you had a laptop instead, as the small display and lack of plug-ins makes the experience rather limited.
With the right apps, you can do a lot more with the iPhone. There are four I find particularly useful:
The first is a Dictionary.com's free dictionary and thesaurus. Sure, you can get that functionality using the web browser, but as a writer, I sometimes need them when the browser's unavailable, such as when riding the subway or visiting another country. And since the whole dictionary app's less than 40 MB, it easily fits on my 8 GB iPhone.
The second is Amazon's Kindle app. The app's free, and more importantly, there are lots of books available for free as well. I've been reading a collection of Robert E. Howard's stories (which doesn't appear to be available anymore) and Twenty Thousand Leagues under the Seas.
The third one is the only one I've paid money for, Documents To Go by DataViz. This program lets me edit Word and Excel documents, and synchronize them with my desktop. I can't touch type with it, and it can be difficult selecting for cutting and pasting, but it's the only app I've found that lets me write on my iPhone.
And finally is my most recent, and simplest, free application, John Haney's Flashlight. All this does is turn the iPhone's screen on a solid color, and leave it on. You decide the color (white's best for pure illumination), and how long you have it stay on (I have it set to 10 minutes). This provides enough illumination to see, and it stays on long enough for me to use it (unlike the standard setting for the iPhone, which keeps the screen bright for less than a minute). You can adjust the brightness as well. I don't find that it's as good as a real flashlight when it comes to focused illumination, but it provides a good diffuse light when you need to find your way. You can also set the app to strobe, which I can see being useful in an emergency situation.
Finally, there's what I keep on my keychain. Aside from keys, I like having three items: a flash drive, a knife, and a flashlight. SanDisk's Cruzer Titanium is a nice one for the flash drive, since its metal casing is solid and the USB plug retracts rather than relying on an easily losable cap. I keep the flash drive mainly so I have a backup of my stories (I did mention that I was a writer, didn't I?) and other important data with me at all times. The one problem is that the key ring is kind of flimsy, and I've had to replace it. The knife and the flashlight may seem redundant considering my other tools, but they're the two things I consider most necessary, so redundancy is a feature, not a bug. I like having them both to-hand in case I lose or have to leave behind my other tools--or just want easier access to a more effective flashlight or a smaller knife. Of course, if I can combine the two, that's even better, as long as I don't need the flashlight on a plane, so I just got this combo from Amazon. It's a bit large for a keychain attachment, but not excessively so. The LED is bright, and has an auto-shutoff after three minutes, which is useful since I've often had LED flashlights come on in my pocket and quickly run down their batteries. The knife is small, but sharp and sturdy. It opens easily with one hand, and locks in the open position. It doesn't lock in the closed position, so it could conceivably catch on something and come open unintentionally, which I'll admit is my biggest concern. So far, though, it's a useful addition to my collection.