As a science fiction writer, he was one of the big three -- along with Isaac Asimov and Robert Heinlein. He is no doubt most famous for 2001: A Space Odyssey, but my personal favorites are "The Nine Billion Names of God" and Rendezvous with Rama (just the first one; the sequels didn't live up.) As a scientist, he will be best remembered for his contribution to the idea of placing communication satellites into geostationary orbit.

In futurist circles, he will long be remembered for his three laws of prediction:

1. When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.

2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.

3. Any sufficiently advanced technology is indistinguishable from magic.
   

I am especially fond of the second one. I hope to live to see the third.