I found this site recently and really like it, as it provides a level of data that usually requires a paid subscription. www.flightaware.com You have to register, but registration is completely free and the site is very easy to use.

It provides real time flight tracking with map for all IFR aircraft (including private aircraft) in the US and provides the filed flight plan route and estimated time of arrival and flight time, as well as groundspeed and altitude data. Coverage is limited to the US, but flight plan route data is provided for overseas flights if you often wonder where your flight is passing over. It also provides METAR and TAF weather information and a summary of all traffic to and from a specific airport. There's also a NEXRAD weather overlay provided on the maps. Perhaps one of the best features is the 90 day archive of arrival and departure times with total flight time displayed; it's good for checking whether a flight has a tendency to run late.

The downside to the flight plan data is that it is provided in coded format, so if you don't know that VXV is the VOR at Knoxville or have a high altitude IFR chart, the information will be difficult to comprehend, but then again, there's a map that you can check after you land to see where you've been.

If this has already been posted, sorry, search was down.

One caveat: I've noticed that it displays the filed route, not the actual route.