nice easy question.
It could stand for any of the following:
Security Support Provider Interface (Microsoft)
Security Service Provider Interface (Microsoft)
Society of Satellite Professionals International
Steady-State Plasma Insulin
Signs and Symptoms of Psychotic Illness (rating scale)
Salle de Soins Post-Interventionnels (French: Post-Interventional Room Care; recovery room)
Soldier Sensor/Platform Interface (ITT)
Apart from the quoted list available at TheFreeDictionary.com, other possibilities which may amuse people interested in future power supply technology, a corporation making use of mathematical information theory, exploitative pseud-science, spookery, or injecting things into people are
Space Solar Power Institute
Statistical Signal Processing Inc
Spirit Search Paranormal Investigations
Single Source Processor (IMINT)
Schmidt Sting Pain Index
If there's a genetic chemistry biologist amongst the readers, maybe he can tell us what sspI stands for when it's a restriction enzyme - I can't, I don't understand that stuff at all: the molecular structure of anything more complex that aniline is way beyond the ken of anyone like me, and I don't know what any of it's called (I can just about remember what RNA stands for on a good day, or even DNA on a very good day).