How about big data itself, the NSA? We know from painful experience that they collect far more data than they can analyze. Having an AI crunch through all of those calls looking for keywords to forward to a (puny) human is already happening on a smallish scale.
What happens when FB, GOOG or your local ISP starts filtering results based upon a government diktat? Oops, already happening, and we are seeing the signs that private filtering is starting.
Security through Anonymity is fast becoming a thing of the past, even without a malevolent AI.
The ability of the NSA to analyze data would no doubt be surprising; their systems must already have implemented AI to process the worldwide data stream. I'm sure it would shock everyone, not least because its scale is so far beyond an individual human brain to manage.
The Folding@Home project is (was?) a voluntary program, where people installed software gadgets on their systems to use background CPU time, then eventually GPU time, to crunch numbers for that project.
A similar project exists for SETI data, and probably others.
I've always been amused by the claims that a given encryption method would require more time than the age of the universe to crack, considering how simple a job like that can be attacked using multiple CPUs, GPUs, virtual machines running on multiple hardware, etc.