Algodoo

Generator will spawn only at given _spawnFrequency and only if number of particles in scene is below _spawnLimit
The number of particles spawned per cycle is set by _spawnNumber and variables of spawned object can be adjusted in _spawnedObject

In this example we can see that all activate if there is not enough particles and slowly turn off as it approaches limits, which can be used for simple regulations

Examples of use: Steam, smoke, fire, projectiles and other particles
Particles doesn't have to be simple at all, you can spawn bombs which will explode, or flying swarm of drones with AI...
Feel free to use, edit, upload, etc. just give me credits in description.