1x04 Phantom Traveler: Dean uses "Dr. James Hetfield". James Hetfield is the lead singer of Metallica.
1x11 Scarecrow: Dean uses "John Bonham", drummer for Led Zeppelin.
1x14 Nightmare: "Father Simmons" and "Father Frehley". Gene Simmons and Ace Frehley from KISS.
3x05 Bedtime Stories: Agents "Robert Plant" and "Jimmy Page", vocalist and guitarist for Led Zeppelin.
3x09 Malleus Maleficarum: Sam as "Det. Bachman" and Dean as "Det. Turner". Randy Bachman and C.F. Turner from Bachman-Turner Overdrive
4x03 In the Beginning: Dean uses "Dean Van Halen". Reference to Eddie and Alex Van Halen from the group, Van Halen
4x05 Monster Movie: As Aerohead said, Agents "Angus" and "Young". AC/DC's Angus Young is the guitarist, not the vocalist. :)
4x06 Yellow Fever: Credit again to Aerohead, Agents "Tyler" and "Perry", Steve Tyler and Joe Perry from Aerosmith.
4x07 It's the Great Pumpkin, Sam Winchester: Agents "Geddy" and "Lee". Geddy Lee is the vocalist and bassist extraordinaire of Rush. Dean also uses "Agent Seger". Bob Seger is an American 'blue-collar' singer.
4x12 Criss Angel is a Douchebag: Dean uses "Agent Ulrich". Lars Ulrich is the drummer for Metallica.
4x13 After-School Special: Dean uses "Coach Roth". David Lee Roth was lead singer for Van Halen.
4x18 The Monster at the End of This Book: "Agent DeYoung" and "Agent Shaw". Dennis DeYoung and Tommy Shaw from the band Styx.
In an episode of season 8, I was surprised to hear them use Lynne & Tandy. Only decently avid fans of ELO would recognize that (Richard Tandy is their keyboardist, my favorite in rock)
