Note: To help refresh your memory, wiki links are provided for characters when available.
Dexter - "Everything Is Illumenated"
Criminal Minds - "Middle Man"
The Vampire Diaries - "Rose"
Fringe - "Amber 31422"
Aaron Ashmore & Shawn Ashmore (twins) guest starred as 'Matthew Rose' and 'Joshua Rose,' respectively. If you watch Smallville, then you'll recognize Aaron as 'Jimmy Olsen.' His brother Shawn was 'Iceman' aka 'Bobby Drake' in the X-Men movies, and he also guest starred as 'Eric Summers' on Smallville.
Smallville - "Ambush"