C-3PO, explains to R2D2 that the thing he is in love with is a fire hydrant. "It is better to have loved a fire hydrant than to never have loved at all" in episode 1396. This is their second appearance, the first one is episode 1364.
Katy Perry, appearing in a skit with Elmo parodying her song "Hot n Cold". The skit was intended to air on the actual series, but the producers overturned their decision due to parent's reaction to Perry's revealing wardrobe.
Lou Rawls, one segment: Rawls appeared in a segment during the first season, to sing the alphabet. He dismissed the concept of using cue cards for the performance, but reversed such decision when he forgot the order of the letters.
Christopher Reeve, three segments: Reeve first appeared on the show in the 1980s. He reappeared in 2000 for two segments; in one Reeve says the alphabet with Ernie and Ernie's Rubber Duckie, the other he demonstrates the independent living skills he developed after acquiring a disability in 1995 (quadriplegia).
Paul Reubens, two segments: Reubens, as Pee-wee Herman, recited his own version of the alphabet in a 1987 insert. Reubens was also recorded for the celebrity version of Put Down the Duckie.
Malvina Reynolds, season 4 cast member: A folksinger, songwriter, and political activist, then 73-year-old Reynolds appeared on the show performing songs "This House Is Your House".
Sally Ride, one segment: In January 1984, the year immediately following her historic mission, Ride taped a segment on the series where she talks about how A is for astronaut, while Grungetta visits.
Kelly Ripa, unable to find the slip of paper on which she wrote the meaning of "frustrated" in episode 4144.
Tim Robbins, two segments, one street scene: Robbins appeared in two 1990 insert segments. In the first, he performs at Prairie Dawn's pageant about the seasons; Robbin's then 5-year-old daughter Eva appeared at the end of the segment. The second, first aired episode 3960, he talks about surprises with Elmo. In a 1997 street scene, Robbins rents a room at the Furry Arms Hotel, which he shares with Muppet animals. Talking to Sesame Street Magazine in 1997, Robbins listed three reasons for appearing on the show: his children, Eva, Jack Henry, and Miles. Robbin's then five-year-old son Miles taped three segments with Baby Bear that appeared throughout the 29th season.
Doris Roberts, two appearances in one special: In the introduction to Sesame Street special The Street We Live On, Roberts is one of the celebrities in the "My Favorite Sesame Street Moments" clip. She later appears in a celebrity montage of the song "Dance Myself to Sleep".
Lillias White, season 21-24 cast member, 1 street scene: White appeared from 1989 through 1993 as "Lillian", a regular cast member of the show, winning a Daytime Emmy Award for her performance. During her stint, she also voiced the lead singer of The Squirrelles, a Muppet singing group, consisting of three squirrels. She returned in 2006 as a guest for a season 37 street scene, playing Gabi and Miles' kindergarten teacher Ms. Walsh, in a flashback sequence.
Robin Williams, six segments: Williams appeared numerous times in the 1990s and 2000s. In the first, aired in 1992 as episode 2963, Williams uses his shoe to demonstrate what makes something alive. Robin then appeared in a celebrity version of "Monster in the Mirror". Another segment has Robin comparing the similarities and difference between himself and a Muppet robin. In another segment, Williams asks Elmo and the kids whether they would allow a nonsense talking, horned two-headed stranger to play baseball with them. In 2000's episode 3684, Williams talks to Elmo about what you can do with a stick. In 2001's episode 3923, Williams shows off the wonders of feet. The Two Headed Monster shows up to play.