A few weeks ago, I ran two Impact Mapping workshop close together. I'm familiar with the ideas behind them, but I'd never facilitated one before. It was great fun, I learned a lot, and I'm keen to run another one at some point soon.
I've posted the workshop outline I used, along with the slide decks, on GitHub.
You can read more about Impact Mapping on the official site. I highly recommend grabbing the book. It's a quick read, but packed full of good stuff.