My favorite is the special they did for the show's 10th anniversary.
The special was hosted by an African-American fella who walked the street as normal activity went on in the background. I think he said something about him being the original Gordon, but he did not look like Matt Robinson. (he also showed an alphabet segment that he did "without the cue cards")
The part I remember best was with the alphabet. It began with the familiar Fat Blue at Charlie's restaurant asking for a bowl of alphab.. er HOT alphabet soup. When Grover delivers the soup and FB starts reciting the ABCs to verify the integrity of the soup itself, they went through the alphabet with a letter from a different alphabet segment in the show's history. Included was:
-- The split-screen routine with Bill Cosby
-- The Fonz from Happy Days! I think he was in the Cunningham's living room at the time, and I believe Richie was next to him there. Fonzie appeared in a number of other segments in those days.
-- A kid holding Snuffy's snuffle while both are near or next to Big Bird's nest. They were on the letter J if I remember correctly, and they both recited the letter.
After the letter Y, they're back to Charlie's restaurant at the point where FB says the Y is missing when he really means the Z. The film then cut to a live old-west scene (the Gunsmoke set?) where a cowboy character is galloping on a horse toward the camera. The man then jumps off the horse, walks a few paces, then just utters "Z" as a letter appears onscreen. (does anyone remember the rest of that? Was he doing a blacksmith routine for the other letters?)