I've been using FlightRadar for years and have the app on my iPhone...great for looking planes up, with the app you can even put it into Augmented Reality mode where you hold it up to the sky and it puts a label against the plane in the sky making it even easier to identify. Its quite weird realising where a plane is going to and from more or less over Chesterfield....not as many currently obviously but spotted one going from Tel Aviv to Montreal earlier this week in fact there are always quite a few heading to or from North America.
I've also used it to track family and friends when i know what flights they are on, mate once messaged me whilst on his flight to Perth using the wifi on the plane so i sent him a screenshot back showing i knew where he was!!