YourFonts.com is a free online font generator that lets you make an OpenType font (Mac, Win, Linux) from your own handwriting. Fonts generated by YourFonts.com can be further improved with the associated High-Logic FontCreator.
Update: There's also, FontStruct, a free font-building tool by FontShop that lets you create fonts constructed out of geometrical shapes arranged in a grid pattern, like tiles or bricks. FontStruct then generates TrueType fonts for Mac or Windows.
Update 2: via Make Better Media is Flipping Typical, a possibly useful website that shows a phrase of your choosing in the popular typefaces you have on your computer.