Yes, it is possible to slow down light in many ways. They've been mentioned above: send it through some media, like water, glass or air, etc.
If you read Einstein's 1905 paper, you will see that he ONLY mentions the speed of light being a "universal constant" in the "IMAGINARY" universe he created by having all clocks in his IMAGINARY universe be synchronous. There's a highlighted and annotated copy of the paper at this link: http://www.ed-lake.com/Einstein_1905_relativity-annotated.pdf The key passages are on page 3.
The speed of light only has an UPPER limit. As I see it, that upper limit is 299,792.458 kilometers per second in a STATIONARY spot in our universe. But, due to time dilation and the fact that the Earth is FAR from being stationary (it's rotating on its axis, while orbiting the sun as the sun orbits around the center of the Milky Way Galaxy), it can be argued that the speed of light is slower here because the length of a SECOND is longer here.
We also measure the speed of light on earth as being 299,792.458 kilometers per second, but that is only because our seconds are longer.