Find where your Twitter followers located on a map. Map them by Country, State or city.
I found this nifty map via @mashable this morning. It is suggested that it can help determine the success of social media campaigns, which I suppose is possible if your campaign targets a specific geographical area in some way. I'm not sure how I will use the information from my own map but it is interesting to see and possibly track over time as way of examining the elusive "reach" metric.