Pulsus is a low-key, minimalistic game of particles. It is marketed for iOS these days (both for the iPad and the iPhone/iPod touch), but its original Adobe Flash version is still online, available free to play.
Pulsus has a very lean aesthetic, and shuns the flashy graphics many modern games strive for. In many ways, Pulsus reminds us of Auditorium, another Flash game. Like in Auditorium, there is a particle emitter, goals which the particles must hit, and forces that you need to manipulate to make this happen.
The emitter, forces and goals all look like round discs. The particles are tiny colour-coded dots shooting in all directions from the emitter.
Initially, all forces are tucked away in a small bar at the top of the screen. To put a force on the field, simply drag it out of the bar and drop it wherever you want to place it. If it is grey, it will repel all particles, so you can use it to push them where you want them to go. If a force is coloured, it will attract all particles of its colour and repel all others.
Goals, too, are colour coded. Much like in Auditorium, many particles of the correct colour must hit the goal very fast for it to fill up. However, once the goal fills up, it will remain full and you can shift the forces around so the particles are diverted to other goals.
You can play the game in either normal or casual mode. In normal mode, the emitter has a finite supply of particles. You must act fast to divert the flow to where it should go, before the emitter runs dry. Once you complete a level, your score is the number of particles still left in the emitter (the higher it is, the faster you were).
Casual mode gives you an unlimited supply of particles (the emitter is never exhausted), but it is enabled only for levels you've completed in normal mode.
Pulsus author Anthony Mattox included a PDF on the game website, showing how the game was developed from an early paper-based mock-up all the way to the version you see on the web.
The document also credits a number of precedents, including Soda Constructor, Laser Logic, and Spaced Penguin.
Interestingly, Auditorium is not credited because Mattox found it the day after he finished the first version of Pulsus.