Kodama, or Tama for short, is a quirky little ball-toss game that had me quickly hooked, and it wouldn't let me go until I had conquered its final challenge... with extreme prejudice.
The gameplay is simple: you toss a little metal coin into a red or blue scoring area, while avoiding the black areas that will cost you one of your four lives. The screen is divided into two fields: a green field from where you grab the coin and toss, and the little maze-like area that you need to toss the coin into.
The first two levels are so simple that you literally can't fail, but after that the difficulty ramps up. It's usually fairly easy (with a couple devious exceptions) to get the coin into the 100-point red zones, but the 500-point blue zones are arranged to maximize your risk of failure.
I played this game for hours upon hours over a couple of days before finally beating level 20, which is the final level. I was obsessed with completing it from almost the first moment I played.
The graphics are simple to the point of being crude, but that doesn't affect gameplay at all. Games like this live or die on the strength of their physics, and the physics in this game seem spot-on. The coin does pretty much exactly what you expect it to do every time. Once you've played for a while, you'll know if the toss is good or not the moment you release it.
There are only four sound effects in the game: two scoring sounds, a sound for failure (think "You've all overbid!" on The Price is Right), and the soft clink of metal on metal as the coin bounces on the walls. There's a subtle effect here too: the ricochet sound's pitch and volume change based on the speed of the coin... a nice, realistic touch.
My only small frustration with the game is the finite number of chances you get to win. You may find yourself (like me) playing through many times because of stupid mistakes or bad mouse movements. I would have appreciated either a certain number of chances per level, or perhaps even ramping up the difficulty all around and giving you infinite tries to finish.
PS: I strongly recommend checking out other games from the same site, which as far as I can tell is simply titled Game (going up several levels to the ultimate root site leads you to Cool & Warm, a Japanese "Game and Design Company"). The games run the gamut from strange to inspired to inscrutable. They're all original, all well-produced, and all fun.