Purple Tinker developed a font based on G4 pony packaging and art: Celestia Medium. Equestria Daily reported, and an anonymous commenter pointed out that the actual font being used is Generation B by Harold's Fonts.
If you're interested in fonts used in earlier generations of pony packaging, check out MLPNews Featured Site Where the Rainbow Ends, which has a page dedicated to G1, G2 and G3 pony fonts. dafont.com has two free fonts that are similar to the G1 fonts (but not exact matches): Grease and Storybook. And JasmineUPC is similar to another font used on G1 backcards.
Do you know of any other pony fonts? Share them in the comments.