Fermcap-S is an anti-foam agent that you can add to your starters, your boil, and your fermenter to help reduce or eliminate boil overs and blowoffs. It's nontoxic and approved as a food additive. It requires agitation to dissolve, so after fermentation completes, it precipitates out and settles down in the trub. Long story short, it never makes its way into your beer, and hence doesn't affect head retention.
How did it fare? I added Fermcap to my boil kettle, along with enough wort to leave only one inch of clearance from the top of the kettle. It worked like a charm. A roiling boil erupted without spilling a drop. Effectively no foam. Magic.
With regards to fermentation, the results were less stark. I also added Fermcap to my 6.5 gallon carboy, and Scott B. added some to his 5 gallon Better Bottle. With plenty of headspace, I observed an approximately 1.5 inch krausen. Scott had effectively no headspace, and hence had some blowoff spilling over his airlock. Scott believes that Fermcap reduces the intensity, but doesn't eliminate the blowoff. I agree with that sentiment.
You can read others experiences here.
