If you cannot stretch to the cost of the latest Airnav Radarbox or Kinetic SBS-1 real time radar receivers then why not try the next best alternative...
Two websites in particular provide an online real time radar display of aircraft that are transmitting ADS-B position information. The first one being RadarVirtuel.com and the second FlightRadar24.com.
Both of these sites are collating position information submitted from many different receiver owners all over the world.
My personal favourite though, has to be flightradar24.com because there are more options to customize the display. For instance, they have recently added the option to display up to three lines of information next to each aircraft displayed on the map. On the first line you can display callsign or aircraft registration, on line two, type and registration or altitude and speed or just one of any of those four options. On the third line, you have all the same options again, so over the three displayed lines you can instantly see a lot of information!
One other point, FlightRadar24.com also provide apps for the Iphone and Google Android operating system phones - so I've now got a real time radar app on my mobile phone wherever I go! In fact, whilst I can't comment on the Iphone app, my Android phone app even has an option called 'AR view' (I think it stands for aspect ratio view) so you can switch to this option and point your phone at the sky and it will display all the relevant info on any plane that's within your 'view' although, in practice, this doesn't always work correctly.
Why not give one of them a try - you'll soon be hooked!