From here, you can download the airport and navaid database files, and a binary or source code distribution of the fplan program. If you are not familiar with the transfer of binary files with a web browser, here are some hints for common browsers. You will need to download one of the database files and either the binary or source code distribution.
The pipeline from the US National Airspace Data has been restored! Unfortunately, the NASD data is no longer available for free download. In view of the fact that the free data was never current, this isn't really a significant loss. The good news is that the National Airspace Data can now be obtained on an ISO 9660 CD-ROM for a nominal fee by sending a written request to the FAA Aeronautical Information Services Division. The data is valid for 56 days starting on the effective date specified for that publication cycle. For ordering information, see the fplan FAQ. I will be periodically purchasing a copy for my own use, however I can't guarentee the data here will always be current. You will need to read the following disclaimer before downloading the database files.
Thomas Wimmer has constructed fplan database files for Austria and Germany. At this time, database files are not available for other countries. However, the next release of avdbtools will provide support for Digital Aeronautical Flight Information Files (DAFIF), available from the US National Imagery and Mapping Agency (NIMA). These files were once available by CD-ROM on a subscription basis only, but are now available for download by FTP at no charge. The DAFIF database files include information for many major airports outside the United States.
The development source repository is accessible by anonymous CVS.