I've used AirNav Live Flight Tracker for years and it by far the best I've tried.
Cheap, easy to use and a stand alone software. You can open many views at the same time (for example to monitor LAX and JFK approach simulataneously).
And I've heard they be adding UK airspace soon.
Their website is at:
And BTW: they are the major sponsor of airliners.net...have a look at the bottom of any airliners.net page.
A free trial is available from their homepage.
Sorry, but I have to disagree. Airnav is the worst one of them all. I've said this before in another 'flight tracker' thread. Points of departure and destinations rarely show, aircraft types rarely show and planes mysteriously vanish off the map and suddenly re-appear a few hundred miles further into their journey. Also, if you rattle up a particular airport for arrivals and departures it only lists about 75% of what's actually due.
For example, the AAG709 Electra I reported a few days ago did appear on the map (luckily, else I would have missed it) but didn't show in the St.John's arrivals, where other Iceland-Canada bizjets and ferry flights were doing.
In my opinion it's very temperamental and patience is required with it.
Be advised that the 'free trial' isn't actually correct. The 'free trial' consists of using old, saved flight information - so in essence it's a 'demo' not a 'free trial' as none of the flights are live.
Personally, I use FBOweb's $9.95/month package which I reckon is the best by far. It isn't very good on European flight coverage and that's where I use flytecomm.com. FBOweb also has a flight archive which I find handy to see what I've missed while working away, as does Flytecomm, but Airnav doesn't.
FBOweb is the best for bizjet coverage too. If they don't show it, none of them will.
Hope that's of help.