Re-watching the series and, welp, can't JUST watch.
  1. Wil Wheaton
    s03e05, s03e19, s04e08, s05e05, s05e22, s06e07, s07e10, s07e19, s07e23, s08e20, s09e07, s09e11, and s09e17 as himself
  2. Christine Baranski
    s02e15, s03e11, s05e01, s07e04, s07e24, s08e23, s09e17, s09e23, s09e24, s10e01, and s10e18, as Leonard's mother Dr. Beverly Hofstadter
  3. Laurie Metcalf
    s01e04, s03e01, s04e03, s05e06, s07e18, s08e23, s09e01, s09e24, s10e01, and s10e12, as Sheldon's mother Mary Cooper
  4. Josh Brener
    s05e10 and s06e16, as Dale
  5. Charlie Sheen
    s02e04, as himself
  6. Octavia Spencer
    s02e05, as an employee at the DMV
  7. Riki Lindhome
    s02e06, as Ramona Nowitzki
  8. Analeigh Tipton, Samantha Potter, and other ANTM contestants
    s02e07, as themselves
  9. Sara Rue
    s02e08, as Dr. Stephanie Barnett
  10. Michael Trucco
    s02e11, as Dr. David Underhill
  11. Summer Glau
    s02e17, as herself
  12. George Fitzgerald Smoot III
    s02e17, as himself -- astrophysicist, Nobel laureate
  13. Lewis Black
    s03e02, as Professor Crawley
  14. Steven Paymer
    s03e16, as Judge J. Kirby
  15. Stan Lee
    s03e16, as himself
  16. Judy Greer
    s03e21, as Dr. Elizabeth Plimpton
  17. Steven Yeun
    s03e22, as Sheldon's former roommate
  18. Steve Wozniak
    s04e02, as himself
  19. George Takei
    s04e04, as himself
  20. Eliza Dushku
    s04e07, as FBI Agent Angela Page
  21. Neil deGrasse Tyson
    s04e07, as himself
  22. Eric Andre
    s04e08, as the guy controlling the line at the cinema ("the petty functionary with the clipboard, bitch")
  23. Keith Carradine
    s04e09, Penny's father Wyatt
  24. Rick Fox
    s04e13, Bernadette's ex Glenn
  25. LeVar Burton
    s04e17, as himself
  26. Brian Greene
    s04e20, as himself
  27. Katie Leclerc
    s05e04, as Emily
  28. Brent Spiner
    s05e05, as himself
  29. Blake Berris
    s05e09, as Kevin
  30. Mike Massimino
    s05e15, as himself
  31. Peter Onorati
    s05e18, as Angelo
  32. Leonard Nimoy
    s05e20, as Spock
  33. Stephen Hawking
    s05e21, as himself