Really cheap things? Pilot G2 pens, metal water bottles from lost and found at my gym (they put them up for grabs just before donating them), the carabiner I use as a keyring (got it for free at a county fair years ago), Google voice/Google hangouts, which allow me to make and receive calls for free anywhere in the world I have Wi-Fi, and have saved me around a thousand dollars so far.
Less cheap but also ridiculously useful: My Honda fit, which has carried everything I've ever tried to get inside it, my custom sliding car bed for the fit with sleeping pad and various sleeping bags appropriate for the weather, which has saved me thousands in rent, my galaxy s3 which is also my e-reader (going to get a kindle when they refresh their line though, 10+ books on a cellphone screen in a year is a bit silly), my 1 pair of quick dry pants and 1 quick dry shirt, which I wear probably 150 days a year each.
Honestly, I feel like I live a pretty damn mustachian lifestyle as far as my possessions go- there's not much I own that I don't get a lot of use out of.