Note: To help refresh your memory, wiki links are provided for characters when available.
Chuck - "Chuck Versus The Fear Of Death"
Summer Glau guest starred as 'Greta.' She is well known for her roles as 'Bennett Halverson' on Dollhouse, 'Cameron' on Terminator: The Sarah Connor Chronicles, 'Tess Doerner' on The 4400, and 'River Tam' on Firefly. She will be playing the role of 'Orwell' in the upcoming NBC series The Cape.
Lie To Me - "Beyond Belief"
Julia Campbell guest starred as 'Carol Ashland,' the mother that attempted suicide at the beginning of the episode. If you're watching The Event, you may recognize her as 'Val Buchanan,' Leila's mother who was shot when her sister was kidnapped and father was coerced to fly the plane. She also played the role of 'Sally Mitchell,' Trinity's wife on Dexter.
No Ordinary Family - "No Ordinary Mobster"
Amy Acker guest starred as 'Amanda,' George's co-counselor. She's well known and very recognizable from her roles as 'Katherine Walters' on Human Target, 'Rachel Conroy' on Happy Town, 'Dr. Claire Saunders' & 'Whiskey' on Dollhouse, and 'Fred' & 'Illyria' on Angel.
Human Target - "Ilsa Pucci"
Tahmoh Penikett guest starred as 'Peter,' Mrs. Pucci's bodyguard. Dollhouse fans will recognize him as 'Paul Ballard.' He also previously guest starred on Smallville as 'Vince Davis' and 'Wes Keenan.'
Community - "Conspiracy Theories and Soft Defenses"
The Good Guys - "Supercops"