Source code under the GNU GPL or binary versions that allow you to create fonts under the SIL Open Font license.
5 USD or more
The latest version for commercial fonts and upgrades for one year.
The latest version is birdfont-2.18.6.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.