First off lets go over the most commonly used cables your likely to come across, and look at some helpful accessories which should help you ensure your ready to charge wherever and whenever.
Micro USB and Mini USB
Unfortunately device manufacturers are always fighting over competing standards for their cables. Luckily two main standards now power most Android smartphones, and connect a wide range of other computer peripherals. Pictured above we have the Micro USB and the Mini USB (confusing I know). One of each of these should cover most non Apple USB powered devices, and help you connect everything from USB card readers to small external hard-drives.
USB Printer Connector
This cable standard, often used in printers, external hard-drives and pro audio equipement is a bit less common, but still very helpful to have. I don't keep a printer in my home, but have several of these for pro audio devices, and have often come across random devices that share the standard. Add at least one of these to your kit and you'll likely be pleasantly surprised when it comes in handy.
Apple 30 Pin connector
Speaking of Apple, both the iPad, iPhone and iPod have used the same USB cable standard over several generations. You likely have a couple of these white cables lying around, and with the popularity of Apple devices, adding them to your kit is a definite must (even if you don't carry an iPhone yourself).
Micro USB to 30 Pin Converter
As a side note, you can also pick up these handy Micro USB to 30 Pin converters, which let you connect any 30 pin device to a Micro USB cable. Very handy in a pinch, and especially helpful in keeping your cable kit lean.
USB Power Adapter
If you own an iPhone, or an iPad you definitely have one of these lying around. In fact most smartphones likely ship with one. Simple enough, you have a power plug on one side and a USB port on the other. Cross compatible with any device which can charge off USB, toss in a couple of these in your kit for easy charging wherever you can find a free plug.
Put your kit together
Narrow you USB cable kit down to just what you need, (likely pretty close to what I've listed above) and depending on your use case, find a smart and clutter free way to keep the cables together in one place. The idea here is to create a spare set of cables to supplement your main use cables, so you may consider packing the cables so their quick to access and maybe even ready to travel with.
This might be a compartmentalized container, like a tool box, or a drawer organizer. If you do plan on taking the kit on the road consider using a fanny pack, or even an old discman case or camera bag might work. They key is to choose something with compartments so you can keep the cables separate and untangled.Having a spare set of cables, which your confident you can use (do your own research on the cables in your collection to make sure) makes it easier to throw away all those extra device cables you'll never use. Every once and a while I'll find an old cellphone (pre-smartphone) power supply lying around. If I can, I'll match it to the device and donate it to a thrift store. Otherwise, it's time to toss out that cable and only keep a small, accessible, collection of multi-use cables I know I'm going to need.
(Images: Sean Rioux)