The FiberFix spoke works well, is easy to install, and packs up small. It's a bit pricey and doesn't do you much good if you break more than 1 spoke though.
I think a better solution is to carry some spare spokes (I carry mine taped to the seat tube) and a Hypercracker or Stein tool. I had to replace 2 spokes on my last tour (combination of cheap wheel, heavy load, and bad roads) and managed to do so in about 45 minutes with the Stein.