Pixelfont is a neat web tool that will take an image you provide, laid out in the proper format (which you have some control over, like character width and height) and will turn it into a pixel TrueType font for you to use! The gamedev applications of this should be obvious.
This isn’t the first free online font-building tool of this nature. The classic in the field is Fontstruct, which can also produce pixels that aren’t square, and can even extend outside of their cells, but also shows ads (although unobtrusive ones) and doesn’t let you import an image. Still, both are rather of use, or at least are fun to play with!