He was primarily known for his syndicated "American Top 40" pop countdown, but he also did voiceover work including Shaggy in the Scooby-Doo animated franchise and Alexander Cabot III in the Josie and the Pussycats cartoons. Kasem also did some narration behind some animated segments that debuted during the first season of Sesame Street. Here's the best known one, "advertising" the letter B. You may remember this as one of the segments watched by Drew Barrymore in E.T.: The Extra Terrestrial.
http://www.youtube.com/watch?v=nyqesU0Jdto
Remember what I wrote a couple entries back about the first decade of Sesame Street having a somewhat anarchic quality about it? :)