When you go way off the beaten path, it's nice to know your bike can bring you back. The 9 speed PowerLink is great to keep in your seat bag or Camelbak... just in case.
With a chain tool and a PowerLink you'll be able to complete almost any trail side chain repair. If you need an easy to carry chain tool check out the Park Tool CT-5 Mini Chain Brute here.
Great for quick removal and reinstallation, and absolutely invaluable for emergency trailside repairs.
I keep a couple in each bike's tool bag as well as in the bike toolbox at home. They weigh next to nothing and don't take up any space (put them in a plastic bag so they don't migrate down to the bottom of the bike bag). Along with a good chain tool they can save a ride for you, one of your buddies, or some poor soul stuck on the trail.
Last summer I was an hour into a ride on the Maah Daah Hey trail in the North Dakota badlands. As I rose out of the saddle near the top of a steep climb, my chain snapped. Because I had one of these and a chain tool in my pack I was back on the trail in a few minutes. Otherwise I would have been pushing my bike back to the trailhead.
What is there to say? They work, they're huge time savers, and they're inexpensive.
If you ride with a compatible chain, be sure to buy a 5 pack, and keep them in your pack while you ride, they are the fastest way to deal with a broken chain on the trail!