Originally Posted by cpach

I like using a lightweight wallet all the time because I am too lazy to remove my ID and some cash before riding, and am afraid that they're more likely to get lost that way. Right now I have a flowfold wallet that I think is literally partially made with carbon fiber (part of recycled sail cloth) and weighs 16g: http://www.flowfold.com/product/ares-billfold. The only disadvantage is that it doesn't breathe at all, so when commuting in jeans my back pocket gets slightly sweaty. I previously owned http://www.simblissity.net/litefold_b-line.htm, which is a similar product. I like the flowfold better, although the simblissity version is half as heavy (8g!)

I've also carried a derailleur hanger at times, especially off road. A spare master link or pin is also a good idea. Fig newtons are my favorite thing to shove in a pocket and snack on.

Smartphones are pretty useful. Aside from Strava and checking directions, I once read a chapter of an ebook while waiting at the bottom of a hill my friend was descending like a wuss.