Just some thoughts:

Environmental Datasets

As stated by the other comment, have a look to weather, forecast (or similar services).

Space data

What about data from the Universe?

Flights

Here’s some real time flight tracking data: the evaluation plan is limited but free.

Social Networks

Twitter Streaming API, Facebook RealTime Updates API (in case you hyave already an app on the social Network), Foursquare

Energy production

Various energy production (real-time and quasi real-time) data.
On these ones probably you have to fiddle around the pages to track the sources of the data, sorry.

Website traffic

Google Analytics RealTime API (for your registered websites)

Money

Bitcoin transactions, Stock Market API

More?

And here and here an “not-so-updated” list of other Real-Time API services (some free, some others not).