Before they were famous, they were on Veronica Mars

WHO: Leighton Meester
CHARACTER: Carrie Bishop (2 episodes)
WHERE ARE THEY NOW: Prior to V.M., Leighton was an average actress who had many small roles on television series like 24, 7th Heaven, and Entourage. It wasn't until her main role in Gossip Girl when Leighton became a well known A-list actress. Since then, she starred in many movies like The Roommate, Monte Carlo, Country Strong, and Date Night.

WHO: Dianna Argon
CHARACTER: Jenny Budosh (3 episodes)
WHERE ARE THEY NOW: Dianna is on Fox's hit show Glee. She plays a cheerleader who eventually turns into a rebel. Dianna also had a major role in the movie I Am Number Four.

WHO: Max Greenfield
CHARACTER: Deputy Leo D'Amato (11 episodes)
WHERE ARE THEY NOW: Much like Dianna, Max is also on a hit show on Fox called New Girl, starring along side Zooey Deschanel. His character is Schmit, an OCD cardigan wearing metrosexual. Max has also been on shows like Ugly Betty, Gilmore Girls, and Greek.

WHO: Adam Scott
CHARACTER: Mr. Rooks (1 episode)
WHERE ARE THEY NOW: Adam has been in many projects ever since. He's been in Step Brothers (where he played Will Ferrel's asshole brother), Knocked Up, Our Idiot Brother. With his television work, he's been in Showtime's Party Down, and NBC's Parks and Recreation as Ben Wyatt.

WHO: Jessica Chastain
CHARACTER: Sarah Williams (1 episode)
WHERE ARE THEY NOW: 2011 was a huge year for Jessica, as she was in 7 movies. She's earned many award nominations for her roles in The Tree of Life, The Help, The Debt, Take Shelter, and many more. Within 1 year, Jessica went from an unknown face to an A-list movie star.

WHO: Michael Cera
CHARACTER: Dean the tour guide (1 episode)
WHERE ARE THEY NOW: Previous to V.M. Cera was already a well known actor in the short lived series Arrested Development. However, it wasn't until after V.M. that Cera really became an A-list actor starring in movies such as Super Bad, Juno, Nick and Norah's Infinite Playlist, Year One, Paper Heart, Youth in Revolt, and Scott Pilgrim Vs. the World.

WHO: Amanda Seyfried
CHARACTER: Lilly Kane (11 episodes)
WHERE ARE THEY NOW: Shortly after V.M, Amanda got the role of a life time playing Karen Smith in Mean Girls. After that she had starred in many big budgeted movies like Mamma Mia, Jennifer's Body, Chloe, Dear John, and Red Riding Hood. Seyfried is arguably the most successful actor to come out of Veronica Mars.

SOURCE: My observations, Google, Imdb, the internet