Birdfont is a free font editor which lets you create vector graphics and export TTF, EOT and SVG fonts.
RoboFont is a UFO based, mac only, font editor. Written from scratch in Python with scalability in mind, the editor allows full scripting access to objects and interface.
FontStruct is a free, font-building tool sponsored by FontShop that allows you to easily create fonts using geometrical shapes. Once you’re done building, FontStruct generates TrueType fonts, ready to download and use in any application.
Creating a prototype for an original typeface, a custom logo for your brand/client or experiment with type design by altering an existing ‘base’ typeface.