Well out of box the phone most likely came with TouchWiz launcher and framework. So as I said if you want completely clean that is called ASOP. Which is pure android without anything extra.
You can get CyanogenMod here which is ASOP:
Galaxy S Plus I9001 - CyanogenMod 7.1 and 9.0 [Porting state] update 08/06/2012 - xda-developers
CM7 is 2.3.7
CM9 is 4.0.4
READ ALL INSTRUCTIONS CAREFULLY!