Here's a list of some of my current favorite Seth/Summer fics. Did I miss a great one that you're currently reading? Comment, and let me know! :)
Dark Night, Hold Tight by Saskia Mitchell
This is a future fic, which I usually shy away from in this fandom, but this one's really beautifully written and explores an interest new proposition: a blind Summer. Nice characterization and a fair amount of smut to top it off.
Third Time Around by Somebody's Dark Angel
Set during the season three premiere, this fic is essentially a developed missing end to the beach scene. The foursome decide to spend the night at the beach, and smut ensues for S/S.
Comfort and Contradictions by Spud14
An alternate ending to season one; what if Summer caught up with Seth before he had the chance to leave?
Daddy's Little Super Hero and
Adventures of a Pint-Sized Fashionista Super Hero by gidgetgirl delux
What exactly would a combination of Seth and Summer in child form be like? Hilarious look at a possible future offspring of the duo.