I use flightaware.com
theres also ifly which hooks up with google earth and virtual earth
lots of features with ifly
i've found that flightaware has the best updating times, ifly has a 10 min delay with the data
flightaware, has I think a 4 to 6 min difference, which is reasonable, but it does update every min with the 4-6 min old data
havent come across any sites that are faster than that
looking for faster ones