Sam Vermette of The Transit App wanted multi-colored versions of the little pulsating blue dot Apple uses to show your current location in maps, and Apple doesn't expose a public way to do that. So, he had to rebuild it from scratch. Or, in more technical terms, according to Vermette's blog:

Working on Transit’s next big release, we came across the need to be able to give MKUserLocationView (a.k.a. that pulsing blue dot indicating the user’s location) a custom color. Problem is, that annotation view is private, undocumented and, more importantly, made up of resource artworks, making it very rigid to any kind of visual customization. This called for yet another exciting journey of replicating a stock UI element.

Vermette details the process he went through, and the results he managed to achieve. It's a fascinating read not only for developers or designers working on custom controls, but for anyone interested in just what goes in to making even the smallest interface elements delightful.