Note: All official release aircraft can now be individually downloaded and updated with FlightGear itself. This is a nice way to browse new aircraft and make sure your favorites are always kept up to date.
If you would like to bulk download the entire set, the zip packages are located here: http://mirrors.ibiblio.org/flightgear/ftp/Aircraft/ These can be downloaded an unzipped into your fgdata/Aircraft/ folder to install them.
This aircraft set is sourced from the official FlightGear aircraft development repository and is maintained and kept up to date with the most recent release.
3rd Party Hangars
There are a large number of additional aircraft developed and supported in 3rd party hangars. You can find the most current information and links on our wiki page: http://wiki.flightgear.org/FlightGear_hangars