Mindblowingly cool design of riboswitches, nucleic acid logic circuits, etc.
This page mostly focuses on deliberate attempts to imitate electronic circuit design, and is obviously waaaay incomplete; I haven't even scratched the surface, really.
It's strongest for logic gates and suchlike, and very weak on applications and on the chemical diversity of RNA.
I haven't even gone near DNA nanotech.
(go start a page!)