This page will be extended over time. Please send us any links you think we should add to our list of useful websites.
http://www.airframes.org - Aircraft Registration Lookup
http://www.flightaware.com/ - Live Flight Tracking
http://www.radarvirtuel.com/ - Real-time visual flight tracking over Europe. Based on amateur electronic spotting so doesn't cover all flights and no historical data
http://www.flightradar24.com/ - Similar to the above
http://www.libhomeradar.org/ - Collates information from electronic plane-spotting (can be quite hit and miss regarding data but it might be useful.
http://www.live-military-mode-s.eu - Site for logging military flight data
Amateur plane-spotting sites, mostly in the UK. These are usually based near an airport and will often give details of all planes picked up, including those that just fly over and don't land: