Preamble: I don’t count people, beliefs, etc., as things and as such will only be mentioning actual physical possessions.
2. Swiss Army Knife(SAK). The knife was the first and arguably most useful tool man ever devised. Add in screwdrivers, scissors, bottle openers, etc., and you’ll have the tools to MacGuyver your way out of many problems thrown in your path. I prefer the Victorinox Compact model as it doesn’t weight down my pocket and has the essentials for me(knife, scissors, bottle/can opener, screwdriver, corkscrew). In the past, I’ve had a lot of “everything but the kitchen sink SAK’s(including one with a torch lighter), but now I prefer smaller and more mobile. The Compact is great quality and only US$20 or so.
3. Bandanna. Endless uses for this little square of fabric, but I mostly use mine as a napkin, tablecloth when eating something messy like a piece of fruit at my desk, wrap my ipod in it to protect from scratches, and as a potholder. If I’m outside and it’s hot I’ll pour water on it and tie it around my neck for evaporative cooling. I usually try to find a tissue or paper towel to blow my nose, but if not available, I can use the bandanna. Of course, I don’t do any of the other uses with it until after it’s laundered in that case. Since I have 5-7 of them, I get into a daily rotation. Also, it was a lifesaver in one instance of a roadstop bathroom not having any TP. I’m glad that one was a 99 cent walmart cheapo, ’cause there was no way I was keeping it afterward.
4. Wok. I love my grandmother’s cast iron skillet, but a wok is the most useful piece of cookware ever. In addition to stir fry, I’ve made eggs, soups, curry, fried chicken, and just about any type of cooked food can be made in a wok. I have a carbon steel wok(got it at a thrift store for US$6!), but one day I’d like to get a cast iron wok and lid with no wood parts so I can use it as bakeware. Also, if you get into the wok, make sure you get the necessary accountrements: lid, spatula(s), steamer basket, etc. http://www.thewokshop.com/ is a great resource.