Particle Playground can do some amazing things but it is difficult to learn, only 2d (though it can fake 3d) and not the same. There are only a few Particle Playground tutorials out on the web. A quick Google Search brings up a fist full of places to start.
OK, what would be the "real 3d" particle systems softwares? I'd rather they were "particle only" apps, not full 3d suites (of which I'd only use the particle subsystem)?
And thanks for the answer.
There are actually a few particle effect plug-ins included with After Effects. See this page for a list.
There are a lot of great particle plug-ins for After Effects. Trapcode Particular is very popular, full-featured, and well supported with tutorials and such. If the plug-ins that ship with After Effects don't do what you need, then Particular would be a good thing to check out.