You have to assume that Dr. Blob is some kind of mad scientist experimenting on microbia that are so feisty and dangerous that he finds it necessary to mount high-powered weapons around the rim of his Petri dishes so that when they get out of control and threaten to escape, he can unleash awesome energy beams to prevent them entering our environment and destroying the entire human race by infecting us with anthrax leprosy pi or influenzAIDS or something.
Rich Carlson of Digital Eel maintains that it's "based on Conway's Game of Life", and maybe the organism in your Petri dish behaves as cellular automata, but it sure doesn't feel like playing around with Life (which despite the name is not a game at all, but a simple set of rules that produces emergent phenomena--no goals, which, as I've argued, is one of the defining characteristics of the game.)
Instead, well, it's a sort of shoot-em-up--perhaps feeling more like Tempest since you rotate your guns around the Petri dish blasting away. Powerups give you more powerful guns, but the organism evolves more threatening behaviors over time, too, for a pretty compelling, somewhat humorous, but pretty brainless level-based gameplay experience.
Anyway, Dr. Blob's was orginally a for-sale indie game, developed by Digital Eel and published through Cheapass Games, of all things, but in the Christmas spirit (or perhaps because sales have tapered off to the virtually nonexistent), Digital Eel has decided to releases it to the masses for free.
"Free" sounds pretty nice for a game that in 2004 won two IGF awards--for art and audio.