I'm enjoying experimentation with AREDN. Using off-the-shelf equipment, you can build a mesh IP network running in ham frequency allocations. Getting started is a bit tricky at the moment...I've joined the project team to help with some documentation and to do some release testing. Here's a link to a Getting Started guide I'm working on.
If you're interested in playing with mesh networks, take a look at the guide and let me know what you think!