To an extent Aaaaa! (to give it its short name) is typical of all of Dejobaan's output, such that it might be worth covering all of their games with an article (The Wonderful End of the World has already been mentioned here). This is their latest offering, though, and their slickest.
The game was inspired by a level designer showing the team YouTube videos of base jumping, prompting them to say 'we should totally make a game of that' (this story is told far more amusingly within the demo of the game). The purity of concept comes through in the final game, which is straightforward in its simplicity, takes ten seconds to learn, and provides the awesome rush of one perfect activity, perfectly rendered.
The basic mechanic is that huge cuboids float in the air, spiralling downward toward a platform marked with a circle. You jump off the top cuboid and fall toward the platform, steering with WASD and mouselook. Just before you splatter painfully, you hit space to deploy a parachute, and then glide to a stop.
The freefall aspect of the game is the meat. At intense speeds (and with a kickass soundtrack) you plummet past these huge floating tower blocks, which rush up to meet you. Through fine manipulation you guide yourself safely past, but the thrill comes from upping your score - you want to stay as close to a building as possible whilst falling to gain 'hugs' worth a handful of points each, and to cut close to as many different buildings as possible to gain 'kisses' worth many points. Then, when last you fire your parachute, you need to control the wide turning circle of your glide to hit the landing zone, which throws in a little extra bit of finesse at the end of each level. Later on, new elements are added, like spectators who are either fans or unimpressed townsfolk. You use the left mousebutton to wave to fans and the right to flip off the complainers, for bonus points, but you have to get close enough to do it, and you don't want to hit the wrong one by mistake. Yet more mechanics are introduced after that, but each one is so elegantly simple, and the drip-feed of new stuff so well paced that there is no learning curve at all.
Essentially it's a game of two elements -- twitchy, fine manipulation play; clipping the buildings without breaking your bones, and path finding; plotting the best course to the landing platform to score as high as possible (do I plunge past that huge tower for loads of hugs, or do I cross the void, temporarily missing out on points, in order to tag the other towers for more high scoring kisses?). It's a great gameplay mix, and the execution is just so pure that the whole thing is one long thrill.
Having gone this far, it would be remiss of me not to mention the aesthetic. As you can probably deduce from the title, this is an offbeat game. The visuals are a trippy mix of neon-glaring translucency and drab grey skyscrapers, rushing past at high speed in a way reminiscent of Rez and similar. Then there's the text. There's not a bit of writing in the game which isn't snappy and amusing. The excessive and surreal tone might not please everyone, and I could see how it would put you off with its omnipresence if you were against it, but most will probably find themselves heartily amused. There's also a couple of great bits of sound work. First of all, over the title screen, the 'plot' of the game (yes, it has one, sorta), is explained by a female narrator, as if this were the beginning of a movie. Secondly, and perhaps reason to get the demo even if you own the full game, is a hilarious five minute monologue about the inception of the game, which plays over the 'buy this game' screen. (Said screen makes no effort to sell you on the game, either, but instead imparts odd trivia about mortality rates through history.)
The demo contains enough of the game to let you know exactly what you're getting, as well as to provide at least a couple of hours' play in its own right.
(All of Dejobaan's games seem to fit the same little niche as Aaaaa! and all are worth checking out. The oddball, vaguely quake-like Inago Rage is another to check out.)
Update: Aaaaa! is a 2010 IGF Finalist in the Excellence in Design category.