Type light is a free, cut-down version of the Type font editor.

The program can open, save and convert OTF and TTF fonts. Although you need to read the documentation to properly understand how this works (there's a PDF user manual linked from the Help menu). You can't simply browse the Windows Fonts folder and open something, for instance. It's a virtual folder and the program gets a little confused, so you have to copy the font you need to some other folder, first.

And if you're new to font creation, then even after figuring out how to open your TTF of choice, Type light can still be a little baffling. The program makes no concessions to beginners, so if you click Fonts > Metrics, say, you're presented with cryptic values like "WinAscent", "WinDescent" and "x Height"; again, if you don't know what these mean then you'll need to head off to the manual for further study.

If you are more experienced in this area, though (or just have the time to learn), there's plenty to like here. There are simple drawing tools, you can edit TrueType curves, tweak glyph and font metrics, and generally work with all Unicode characters.

And if you need even more power, there's always the full Type version, which has many additional features. The program can import SVG graphics, for instance; there's support for tracing graphics; it can work with TrueType collections (TTC files); and you get more advanced drawing tools, more glyph functions, TrueType kerning and OpenType layout support, action scripts and a whole lot more. The program can be yours for 49 Euros, and there are further details on the Type site.