The airport FIDS sites are good, but it can be difficult to work out just which flight is going over, as often the plane will be departing from and arriving at airports, neither of which is the closest one to you. For example here in Sydney I see flights going over that are departing from Rockhampton and landing at Melbourne. It takes a lot of guesswork then continuing observation to work out what's what.
However, if you grab the latest issue of Silicon Chip Magazine, they have a review of a cool little device that intercepts radar information from aircraft near you, and displays it on your PC. It also connects over the net to get info from all other users of the device around the world, so you can see data from all over. Pretty impressive, but costs $1000.
http://www.airnavsystems.com/RadarBox/index.html
Cheers,
Jason.