On my road/summer commute bike I started carrying a chain tool after an "incident" last year. I could carry a spare power-link but I figured the chain tool might come in handy for any folks having a chain problem on my group rides. I also carry a "fiberfix" spoke which in an emergency can be used as a replacement for any spoke and doesn't require cassette removal if you break a spoke on the drive side of the rear wheel. It's like a thick kevlar string with a cam slide, spoke nipple and tool. It all fits in a small vial.

Doing this though was extra frustrating this past week because I was on vacation in a remote area and riding my mountain bike. I didn't think to bring all the extra goodies on my trip. A bit of chain suck pulled the power-link apart and I couldn't find half of it. D*&mn. I had to wait a couple of days until we made the 30 mile trip into the nearest town. Thankfully they had a bike shop. Lesson learned.