Flightradar24 is a flight tracker that shows live air traffic from around the world. It combines data from several data sources including ADS-B, MLAT and FAA. The ADS-B, MLAT and FAA data is aggregated together with schedule and flight status data from airlines and airports. Flightradar24 apps are available for IOS and Android. This tracker allows for identification of individual flights with altitudes and speeds.
SFO Flight Tracking San Francisco airport has installed an elegant java-based tracking program called Volans. It gives a 3-dimensional view of flight activity for SFO, Oakland and San Jose Airports. Click on the Volans button on the web page and the java app should self install. The information displayed is delayed 8-9 minutes for security reasons.
Flightaware (this link for SFO) is an excellent tracker that includes lists of inbound and outbound flights as well as weather. You can get lots of interesting aviation-related information from it as well.
Webtrak5 for San Jose Airport. They include real time output from noise monitors arranged along the take off and landing paths.
Here’s an incredible interactive view of worldwide flight traffic and a look into the past and future of aviation from The Guardian.com
24 hours of commercial air traffic around the world from 2008 (from Wired)
And 24 hours of commercial air traffic over the US from 2005 (from Wired)