Installable File System?
Internal Field Separator?
Integrated Forecast System?
Internal Family Systems?
International Federation of Settlements?
Innovative Fulfillment Solutions?
Institute for Future Studies?
International Financial Statistics?
Infiniti Financial Services?
Inline Fuel Injection?
Integrated Fluid Servicer?
Iterated Function System?
Irish Free State?
Indian Forest Service?
Indian Foreign Service?
InterFrame Space?
I Feel Stupid?
And my personal favourite which I cleverly just made up:
Internal Fluid Servicer? (High class hooker.)