Once you let someone know you're technically savvy, the chances of you becoming their main point of contact for all tech support increases greatly.
If you've happened to have made your career in a computer-based industry, the problem intensifies. Through the years of being such an on-call tech-support person for a variety of different skill levels and machines, I've amassed quite a bit of gear that I keep on hand to tackle most situations. Here's a look into my recovery toolkit, which will hopefully help you build yours. A warning: not all of this stuff is free, but I've used all of it at one point.
You may wish to create bootable USB drives instead, but there's a few bootable tools I can't do without.
The first is Ultimate Boot CD, a collection of hardware, disk, backup, imaging and diagnostic tools, along with some DOS-like environments, to help during annoyances like firmware updates that still aren't Windows friendly.
Then there's the alternative — Hiren's Boot CD. Previous versions were considerably more useful than the current one, but this was due to using commercial software that it shouldn't have, to expand its abilities.
Memtest86+ is a RAM checker, and invaluable when you've seemingly isolated all other potential issues. When things start going really odd, it's always worth making sure the RAM or power supply hasn't gone bad.
Ubuntu's live disc, in combination with ClamAV, has saved our skins many times. Whether it's the fact that it happily reads and writes to NTFS off the bat, or just provides a secure, bootable environment where files can be rescued, this is absolutely something that must be in your toolkit.
If you can't clear out infections with any of the above, you might also like to have AVG's Rescue CD handy.
I also find Microsoft's Disaster and Recovery Toolkit (DaRT) to be incredibly helpful, which is part of the Desktop Optimization pack.
It provides a Windows-based GUI in which Explorer is made available, you can reset passwords, edit the registry, undelete files, repair MBRs/volumes/partitions, fiddle with the Computer Management snap-in, uninstall hotfixes, and run Standalone Sweeper, which is a version of Microsoft Security Essentials that'll happily download the latest definitions (or accept them on a USB stick if there's no internet connection).
The bad news? It's not free. You'll need a MSDN or Technet subscription, or a licensing deal that is generally targeted at corporations and not the average user.
But be aware that one size doesn't fit all; you'll need a specific DaRT boot disc for each OS.
For recovery software, our go-to programs are in our feature article here, and I recommend that you keep at least one of them on you.
For the truly paranoid, it can't hurt to have a USB drive filled with the latest hardware drivers, from the likes of Nvidia, ATI, Via, Realtek, Marvell and Intel, along with the latest Windows service packs, in case internet access is slow, or nonexistent. For the OS X kind, having a hard copy of OS X can be a life saver.
There are the basics; a one point Phillips head screwdriver, a set of jeweller's screwdrivers, torx keys and hex keys (otherwise known as "Allen" keys).
We've found keeping a 1TB portable hard drive around to be a blessing, with a fast interface like USB 3.0 or eSATA. Keeping a dock handy, which supports both 3.5-inch and 2.5-inch drives on USB 3.0, can also be a sanity saver. From time to time we've also have had to use a USB optical drive.
A 3G dongle or tethered mobile phone can, occasionally, get you out of a spot, when you can get internet access working properly: just make sure your quotas are up to scratch before embarking on a download mission.
Be prepared to walk into a situation where the computer won't even be functioning. Coming with a well equipped laptop could really save your sanity. A PS/2 keyboard has more than once saved me when USB is on the fritz, while a spare USB mouse and keyboard have also had their time in the sun.
It can get more specific, depending on what your non-technical friends need. A tube of thermal interface goop is good to have in case you need to reseat a heat sink, and perhaps isopropyl alcohol, to clean old stuff off. Any number of cables and converters that you may have collected over the years — DVI, VGA, Mini DisplayPort to anything that's not DisplayPort, PS/2, DIN, SATA cables, multiple types of USB cables and Ethernet cables — you name it, at some point it'll come in handy.
So that's my emergency kit, which I've found to tackle most situations. If you've got extra suggestions, feel free to let us know in the comments below — we can always afford to be more prepared.