It's a real time map listing every USGS river monitoring station in the US. You can drill down as far as you want, and get real time readings from any station.
Think about that for a moment, you can get real time readings from any station, anywhere in the us.
So, if for instance, I wanted to get the flow rate and the height of the Red Cedar river as measured at the Farm Lane bridge on MSU's campus over the past seven days, and up to a few minutes ago, I would just have to click here (for a table of half hour depth readings for today, I would click here).
If, as another example, I wanted to get a graph of the height of the Rifle River (measured here) over the past week, I could click here
wow - it may be useless data, but I find this really really cool. You can find anything on the Internet...