I'm a huge fan of Zenware. Pen.io in itself is pretty minimal, but I thought I would have a go at getting even more zen. The result is Pen Zen. Although this project came about simple to test out the new Pen.io API (still in private beta), Pen Zen ended up getting featured on Lifehacker and has proven to be quite a hit.
Theres not a lot to say about the app - its just one text area and two buttons, but for quickly jotting down some notes, its extremely useful.
P.S - developers, want to play with Pen.io API? It not only allows you to post and retrieve Pen.io pages, you can also use it as a simple key store. Think Amazon's SimpleDB but fast, fun, and most importantly free! Send me an email to get early access to the Pen.io API Beta