Water.

I've used USGS real time data before. Before rowing I would check the U.S. Geological Survey website for info on how much water was going over the dam in St. Cloud. The data didn't change much on a day to day basis, but it was interesting to know how fast the river was moving before going rowing on it.

Turns out the same website's water tracking can be quite useful for anticipating water shortages when hiking the Appalachian Trail. Trailplace.com has map-based links to each state's water flow data. Not always useful to the hiker themself, but I suppose they'd be interesting to those who are following a hiker's progress.