Note: To help refresh your memory, wiki links are provided for characters when available.
Mercy - "I'm Not That Kind Of Girl"
Daveigh Chase guest starred as 'Ashley,' the girl who was surprised to find out she had a penis. If you didn't recognize her by face, that's okay. She's looked a little different in her previous roles. She was 'Lilo' in Disney's Lilo & Stitch. She was also 'Samara,' the girl who crawled out of the TV seven days after you watched the video in The Ring. She's also known for playing 'Samantha Darko,' Donnie's sister in Donnie Darko, a role she reprised recently in the sequel, S. Darko.
Criminal Minds - "Outfoxed"
Kristina Klebe guest starred as 'Miranda,' the unsub who was responsible for murdering families where the fathers were away in the military. If you watched Rob Zombie's Halloween, then you've seen her before because she played 'Lynda,' Laurie's friend that was killed by Michael Myers in his childhood home, now abandoned.
Supernatural - "Abandon All Hope"
Mark Sheppard guest starred as 'Crowley,' the demon that apparently is originally the one to drop hints about the existence of the Colt. You may recognize him from his previous appearances on Dollhouse as 'Tanaka' and on Warehouse 13 as 'Benedict Valda.' He also played the role of 'Badger' on Firefly.