Osmos

Another indie game, Osmos. This is a puzzle game where you play as a 'mote', a kind of orb floating in space. The goal of the game is to absorb motes smaller than you, until you are the biggest mote of them all. If you collide with a bigger mote, you will be absorbed yourself. Motes that are smaller than you will be blue and bigger ones will be red, so it's pretty easy to know which ones to avoid.

Most of the other motes are inactive and just slowly float around, but they will also gradually grow in size when they randomly collide with each other. To move your mote, you follow Newton's third law of motion: for every action, there is an equal and opposite reaction. Thus, to accelerate your mote, you will need to sacrifice some of its mass. The bigger your mote get, the more you will have to sacrifice to get it to move. This adds some depth to the game: by moving your mote towards a smaller one to absorb it, you will shrink. Sometimes you will shrink so much that when you get to the smaller mote, you will actually be the smaller one and be absorbed.

A nice thing about the game is that it is possible to speed up the time. This can be very helpful when you're very big and move slowly.
Though the vast majority of the other motes are normal and don't do much, there are some special ones. Attractors will, obviously, attract other motes with its gravity. When it's big enough, other motes can float around it in orbit. Nemocytes will actively try to avoid motes bigger than themselves, so you'll have to chase them down once you're big enough to absorb them. When you collide with antimatter motes, both your sizes will shrink. And there are some other ones like that. These special kinds of motes make the game much more interesting and fun to play.
The early levels are fairly easy, but it does get a lot harder further in the game. It will take many attempts to complete the later levels, which can be quite frustrating. But it makes finally completing them all the more rewarding.

The game is called Osmos, which seems to be a combination of Osmosis and Cosmos. Actually I don't think that the game has anything to do with osmosis, which is the movement of water molecules across a semi-permeable membrane from an area of high concentration to an area of low concentration until equilibrium is reached. The fact that the fluid will go from the smaller droplet into the bigger one is because of LaPlace's pressure law. But ah well. The motes do look a bit like stars in the cosmos.

Website with free demo.