This mod adds a new resource: He-3 (Helium 3)
Forum description: Kerbal Forum
Kerbin needs this resource as fuel for Nuclear power plants, but there is anything of it in Kerbin. We can find it on the Mun (not in Minmus). You can go there and harvest it and come back Kerbin to sell it making a good profit.
This mod uses the same Xenon storage tanks and extractor of the game. Don´t forget to empty it before launch it.