There are probably more than two types, but the two types I know about are injection (into the intercooler) and topical (onto the intercooler fins).
Ken at PG has both.
The injection system uses a water/methanol mix injected into the intercooler to cool down your boost temperatures. The injected solution is combusted during cylinder ignition. This can create a mild steam that will actually help remove and prevent future carbon build-up. Essentially you're creating additional power two says: a more dense boost volume at the same boost pressure, and a cooler combustion chamber helping to reduce detonation.
The topical spray system can use a number of different solvents or chemicals I'm sure. The one that PG has is a liquid carbon dioxide sprayer (like a fire extinguisher). Basically, shortly after being sprayed and warming up from the intercooler and engine heat, the sprayed carbon dioxide will change from liquid to gas and dissipate.
That's what I've learned so far. I'm going to go the route of the water/methanol injection for the sake of not having to pull a bottle all the time and get it refilled. CO2 is just not as accessible for me.
