We use barrel jacks a lot around here and sometimes we need to pop a switch into the mix. That usually means hacking up a perfectly good barrel jack extension cable, but the whole point of using the barrel jacks was so that we wouldn't have to hack things apart... Well, we found a solution: The Barrel Jack Power Switch.
Okay, it's nothing fancy but it is really useful. Plug your power supply into one end and the other into your project and, just like that, you've got a power switch! The on-off rocker switch has a good stiff 'click' to it, so it isn't easy to accidentally switch. It's also fairly small so it won't add too much unnecessary bulk to your project.