http://www.flightradar24.com/
Might not have worldwide coverage as it depends on ADSB users sharing their data, it's also not as lagged as
http://www.radarvirtuel.com/
Personally I use
http://www.coaa.co.uk/planeplotter.htm (independant program) though they don't have fantastic coverage outside of Europe/America.. but their MLAT feature for working out planes which don;t broadcast a position is out of this world