Probably the cheapest and most common silicone of all. As for skincare, it makes the skin silky smooth, creates a subtle gloss and forms a protective barrier on the skin. Also, works well to fill in fine lines and wrinkles and give skin a plump look (of course that is only temporary, but still, it's nice).
There are also scar treatment gels out there using dimethicone as their base ingredient. It helps to soften scars and increase their elasticity.
As for hair care, it's the heaviest of all silicones used for hair care and smoothes the hair like no other thing. It can be a bit difficult to wash out.