Note: To help refresh your memory, wiki links are provided for characters when available.
House - "Broken"
Franka Potente guest starred as 'Lydia,' House's unfortunately-temporary love interest. You may recognize her as 'Marie' from The Bourne Identity and The Bourne Supremacy. She's also famous from a really awesome German movie, Run Lola Run.
RUN LOLA RUN trailer [click here or watch below]
Heroes - "Orientation/Jump, Push, Fall"
Smallville - "Savior"
Brian Austin Green guest starred as 'John Corben,' which if you watched the previews for the next episode and/or know anything about the character from the comics, then you know he becomes Metallo. Green was previously 'Derek Reese' on Terminator: The Sarah Connor Chronicles.
Dollhouse - "Vows"
Alexis Denisof guest starred briefly as 'Senator Daniel Perrin.' He has a history of working with Joss Whedon since he previously portrayed 'Wesley Wyndam-Pryce' on both Buffy The Vampire Slayer and Angel.