Android: When you place or receive a call, Noteify pops up in a floating window with all the information you would need about that contact, almost like an assistant with a cheat sheet for the things you need to know or remember.
Most similar apps only have the option to add a note about the contact, which pops up on call. Noteify has that, complete with rich text editing, but it also has the contact's details from your phone book, so you can edit or access that information if need be. The nice part about this is that you can also edit contact details while on the call.
Noteify is completely free and works just as advertised. It's highly customisable in layout and appearance, so tinker around and set it how you like it.