There are some great designs. One of my favorites is this one. The nerdy people will get it, or at least the computer literate.
I really like this one because there is also a kids' shirt that is a little sprout for if we have more kids later.
So check out Zoey's Attic either on her site or on Etsy for some really great gifts for Dads, Grandpas, and their kids and grand kids!