Spiro is a toolkit for curve design, especially font design, created by Raph Levien.
Mac users: ppedit.dmg contains a toy Carbon build of the app. It is a Universal binary, and (probably) requires 10.4.
There are a couple of ways to get the source code. Developers should probably use the darcs repository. To build from that, use:
darcs get http://levien.com/garden/x3/
darcs get http://levien.com/garden/ppedit/
cd ppedit
make
Alternatively, you can pull a tarball of the 0.01 release. Unpack, then:
cd ppedit
make
The tarball also contains some utilities for Bezier optimization, and for segmenting scans into letter classes, suitable for tracing over.
The code is GPL'ed, with a patent grant for the curve technology. The current version requires Gtk2 and cairo. More details are in the README.
Raph has been using Spiro to design his own fonts for a while. In addition, the plate files for Inconsolata are here.
A web-app version of Spiro will power fontly.
There's a rollicking good discussion at this typophile thread.