George Williams has extended FontForge to support Raph Levien's Spiro curves.
Unfortunately, the process to install everything and make it function correctly is quite opaque to me.
For purposes of licensing and patents and other things I don't understand, the Spiro libraries are not included in the basic install of the latest FontForge, so some other stuff I don't readily understand has to be done to get them all working together.
There is a Gobuntu build recipe at http://understandinglimited.com/2007/10/26/ff-spiro/ which looks like a bunch of UNIX gobbledygook to me.
If anyone knows how to get it working on Mac OS X 10.4 X11, please share with us. I'd be much obliged.