Note: To help refresh your memory, wiki links are provided for characters when available.
Californication - "Slow Happy Boys"
Kevin Corrigan guest starred as 'Mike Zlozowski,' Hank's best friend. He recently guest starred on Mercy as 'Gerald,' the homeless veteran that 'Veronica' tried to help, and he's been guest starring on Fringe as 'Sam Weiss.'
Stephen Root guest starred as 'Ricardo "call me Dickie" Collini,' the husband of 'Sue,' Charlie's boss. He recently played the role of the vampire 'Eddie' in season one of True Blood, but he's probably most famous for his role as 'Milton' in Office Space.
Lie To Me - "Grievous Bodily Harm"
Scene from Snatch with Lennie as 'Sol' [click here or watch below]
FlashForward - "Scary Monsters and Super Creeps"
Dominic Monaghan guest starred (or finally joined the regular cast) as 'Simon,' who mysteriously claims to know what caused the flash forwards. LOST fans will recognize him as 'Charlie.' Lord Of The Rings fans will recognize him as 'Merry.'
Smallville - "Crossfire"
Emilie Ullerup guest starred as 'Catherine Grant,' Clark's first date for the episode. (If you're a fan of the Superman comics, then you know that Cat Grant is is a well-known character.) If you watch Sanctuary on Syfy, then you may recognize Emilie because she played the role of 'Ashley Magnus.'
Sanctuary - "Hero"
Chris Gauthier guest starred as 'Walter,' the guy with the super powers. If you've been watching Smallville, then you recognize him as 'Winslow Schott aka Toyman.' He also starred on Harper's Island as 'Malcolm Ross.'