Calligro is an open source tool for generating bitmap fonts. It uses the same specification as AngelCode's BMFont but unlike other similar programs it's designed specifically to work with characters you draw yourself.

Basically, you export a template, draw your characters in it with any graphics editor, and then import it back to Calligro and generate a bitmap font.

It has both online and offline version. I just released version 2.0 with completely revamped UI.