He is the last, I think, of the greats of his era who remained. With Jack Williamson, he wrote the Reefs of Space/Starchild trilogy; with Cyril Kornbluth, he wrote multiple classic SF novels, one of which, Gladiator-at-Law, I reviewed on my site not too long ago.
He also had been writing a retrospective blog (The Way The Future Blogs) and was still active in the industry.
I never got to meet him, but I knew many of his works. SF has lost one of its longest-shining lights.