Note: To help refresh your memory, wiki links are provided for characters when available.
The Good Guys - "Small Rooms"
Lee Thompson Young guest starred as 'Eric Williams,' the arms dealer that didn't want to sell to the Guatemalans. If you watched FlashForward, then you might recognize him as 'Al Gough,' the agent who committed suicide to change the future. He has also starred on Smallville as 'Victor Stone aka Cyborg.'
Warehouse 13 - "Mild Mannered"
Sean Maher guest starred as 'Sheldon,' the guy working at Loretta's coffee shop who used an artifact to become a superhero. Firefly fans will recognize him as 'Simon Tam,' the doctor with the troubled sister.