The fun folks at Pixeljam have released a challenging new side-scroller called "Potatoman Seeks the Troof," in which you play a little spud in search of enlightenment, traversing a series of difficult terrains, trying to attain it. I've been playing it for a couple of days now, and I've found it good, addictive fun, with a nice balance between level-memorization and hand-eye coordination.
(Thanks, Miles!)