The Wolfram Alpha search engine was recently updated to include real-time flight information and now the company has taken this feature and built it into a dedicated app for the iPhone and iPad. Currently it's only useful for internal flights in the US or international flights that either take off or land in that country.
Nevertheless, it shows great potential as it's one of the most comprehensive flight tracking apps out there - because not only does it offer up-to-date information on flight arrivals and departures, it can also tell you what flights are overhead at any location. You can also search for flights by arrival and departure airport, or alternatively by using either the airport name or its short code. You can even look up current conditions at particular airports and use it to calculate average delay times for specific airlines.
This app shows a huge amount of promise, so let's hope that Wolfram adds support for European flights in the near future.