Solid, if you enjoy particle physics
Osmos is a physics-based game where you control a tiny lifeform. The goal is to suck up the things smaller than you by bumping into them, while avoiding things larger than you. The only way to move around is to shoot off a tiny piece of yourself, pushing you around the level. The game has three main scenarios that provide healthy variety to the game. Depending on your patience, this game could keep you occupied for many hours, because there's even a built in "infinite number of levels" that will let you keep pushing on in your favorites.
The three main scenarios are Lifeform, Ambient, and Attractors. The Lifeform levels have AI opponents that your ultimate goal is to hunt down and absorb. The challenge level for this scenario puts you hunting ever-increasing numbers of organisms.
The Ambient levels have nothing but inanimate objects moving (or not moving) around in a box, and your job is to "become the biggest". There are "Impasse" levels, where you start out very small in a world of huge but non-moving objects, which plays much like a puzzle as you figure out where to shoot your pieces so that you can grow faster than all the other objects. There are also levels with antimatter that will annihilate matter on contact.
The Attractors levels are like big solar systems, with one or more major attracting spheres pulling on you with a constant force, and the other inanimate objects are in orbit around it. There is one version in a box, similar to the Ambient levels, but with attractors that will suck up the other particles. The other two level types are a simple solar system with a single sun, and "Epicycles", which has multiple attractors that act like a sun and several planets. While not modeled with real n-body particle physics, the approximations don't detract from the fun.
If Osmos has a major hurdle to overcome, it is the slow, methodical gameplay. The music is very relaxing, and the levels can take a while to play, especially if you slow down time to be precise. Because you are unable to save in the middle of levels, you can sometimes play for a long time, and realize only later that you can't really win, or you lose because you find yourself moving too fast toward a large body that was out of your field of view. The game allows you to zoom out to view the whole level, but this isn't usually a good view for absorbing tiny bits, especially when you are small. The game does cut down on this some by letting you know if you're too small to win, but even with this notice you sometimes find yourself in unwinnable situations without realizing it. If the slow, puzzle-like gameplay doesn't bother you, Osmos is a good title to add to your collection.
