A good concrete example of the innateness of language (and by extension, grammar) is apparent in any "normal" three-year-old child. I don't get the chance very often anymore [sic], but I do enjoy listening to children developing grammar rules based on what they hear. When they make mistakes by analogy such as "I eated my dinner" you know they've got it all worked out, and now only need to refine.

Edit: Oops, I see I sort of mantled Faldage and Jackie above.