I didn't know that name, but I certainly did know his work. He was the actor/puppeteer who did Big Bird and Oscar the Grouch. Here's a very touching tribute by Big Bird from Spinney's retirement party just last year:
And since What's My Line? is so much my jam, an appearance on that show. Spinney appears as himself at the end of the segment, dressed in a VERY Jefferson Airplane kind of outfit.