An infinitely variable interval timer built around an Arduino
I'm working on grafting some fruit tree saplings, and I needed to be able to provide a cool mist for them at adjustable intervals as they grew. I found timers that could do this over the range of intervals I would need but they were prohibitivly expensive, so I built my own for about $40. It even has a handy lighted LCD display to let you know what it is up to.
Here is a blog post on the details. Please leave me a comment there and I'll be happy to answer any questions or help you get one rolling!