Diver is a stark little game in which you play a small ragdoll model leaping off a cliff. Your goal is to enter the water cleanly (either head or feet first), between two buoys bobbing in the surf below.
The control scheme is quite novel; essentially, mouse position controls the character's stance, and a clean dive involves a precise swooping motion. It's damn hard to get used to, which is part of why the game is challenging -- and also frustrating.
Which brings up an ancillary point that I think Diver demonstrates well: sometimes what makes games compelling is precisely how frustrating they are. The wife of a friend once told me "I know it's a good game when I hear curses from the den;" we want games to challenge us. Not to the point that they're unplayable, of course, but achieving a sense of fiero first requires a sense that overcoming the obstacles we face is a real challenge. A few months ago, I got into a second WoW jag, and was surprised at how easy Blizzard has made the first few dozen levels; they've nerfed the game, in essence. I'm sure they mine their user data extensively, knew precisely what they were doing, and can probably statistically prove that by reducing early difficulty they improve player retention and therefore their bottom line; but I also remember how hard I worked for my first mount. The second time round, it felt all too easy, and I rather missed the sense of thoroughly exhausting the newbie areas before moving on.
Of course, finding the right balance between struggle and a sense of accomplishment is difficult, and Diver may be on the too-hard side because its control system is novel and takes some getting used to. Yet it is compelling, and the sense of mastery as you gain experience is palpable.
It's a level-based game, of course, with some truly scary rocks that are all-to-easy to impale yourself on as the game progresses; and because there's really nothing more here than learning the controls and manipulating the system, it's a fairly shallow game. Yet it's worth attention, for its originality and its interesting use of physics.