Famous Guest Stars

Jennifer Aniston

will return to the small screen as a stalker on the NBC hit, 30 Rock. The Emmy-nominated actress plays a woman obsessed with network exec Jack Donaghy, played by Alec Baldwin. "I think we are all star-struck with her," Jack McBrayer-who plays bumbling page Kenneth on the series-told People of having Aniston on the 30 Rock set.

Olsen follows up her stint on Showtime's Weeds with an appearance on Christina Applegate's ABC hit, playing a bad girl who reminds Samantha of her former self. "She showed up alone, ready to work, is great on set and is serious about the job," Executive Producer Don Todd told People in June. "This is a girl who's been in front of the camera almost literally her entire life. She's smart, and she knows what she's doing."

When Greek's Casey-played by Spencer Grammer-finds herself faced with a problem similar to one Lauren Conrad once dealt with on The Hills, she dreams of having a "what would LC do?" moment with the reality queen.

"Everyone is wondering 'What's she doing?' when they see photos of Lindsay on the set," says actor Mark Indelicato, who plays Betty's little brother Justin. "She plays Betty's high school nemesis, as you saw in the finale, and now she's working at a burger joint. It's so cool that she plays the most unglamorous part ever!"

Nicole Richie

kicks butt-and takes a few lumps herself-when she guests on the NBC spy comedy Chuck. As Sarah's high school nemesis Heather Chandler, Richie plays the good girl gone bad to the hilt. When Heather and her husband-played by the Wonder Years' Ben Savage!-return home for their class reunion, Chuck and Sarah must foil their evil plans.

Though she's receiving plenty of attention as the star of her own show, Leighton Meester returns to Entourage as Justine Chapman, one of Hollywood heartthrob Vince's former love interests. Meester plays a virginal pop star on the cable show-quite unlike the vampy character she plays on Gossip Girl, Blair Waldorf.

Katie Holmes

gets sultry when she belts out "Hit Me With a Hard Note" during her guest spot on ABC's Eli Stone. Holmes stars as an attorney who makes an appearance in one of Eli's (played by Jonny Lee Miller) visions.

Known as a boldface name in New York City, socialite Tinsley Mortimer attempts to make herself a household name with an appearance as herself on the Season 2 premiere of Gossip Girl. At a white party in the Hamptons, Mortimer-who has designed handbags for Samantha Thavasa and recently launched her own line, Riccimie by Tinsley Mortimer-gives Jenny a much-needed compliment about her fashion sense.