Father and Son: STS-1 and STS-135
The picture we waited 30 years to complete. (hi reddit!)
UPDATE 7/14/2011: I've gotten many, many requests for permission to use the photo, so instead of responding to each individual request, I've changed the license on this picture to Attribution-NonCommercial-NoDerivs. This allows you to download it and share it with others as long as you attribute the photo appropriately and do not change it in any way or use it commercially.
Thanks for all the positive feedback. We are humbled by the response!