Source code under the GNU GPL or binary versions that allows you to create fonts under the SIL Open Font license.
1 USD or more
A single version for commercial fonts or fonts with other licenses than the SIL OFL
5 USD or more
The latest version for commercial fonts and upgrades for one year.
The latest version is birdfont-2.6.9.tar.xz. Source code for all versions can be downloaded from the release directory.
Packages are available in many Linux distributions.
A package is available for OpenBSD in graphics/birdfont.