Let's go for the big one right off ... Glenn was Otho in Beetle Juice. Tim Burton, the director of that film, knew a good thing when he found it, and has continued to bring Glenn into his films, right up through the remake of Planet of the Apes. He's also done voices for Lilo and Stitch, amongst others. And in a funny role, he was Typhon in Hercules.
Glenn's Official Website