Nathan Fillion - Mal Reynolds
Nathan Fillion has popped up on various shows and movies since his days as captain of the Serenity, including roles in of course Serenity like the rest of the Firefly cast, plus Slither, Waitress, Desperate Housewives, and Lost. He's also starring in an upcoming midseason newbie, Castle, which we fear may already be falling into the Cancellation Danger Zone even before it premieres!
Adam Baldwin - Jayne Cobb
It's hard to follow a role like Jayne, but Adam Baldwin has found another excellent muscleman role for himself on the beloved Chuck as NSA Agent, John Casey. Other than Chuck, Baldwin appeared in a few films including Drillbit Taylor, had roles on Day Break, Stargate SG-1, NCIS, CSI: NY, and a few other shows.
Gina Torres - Zoe Alleyne Washburne
Gina Torres has shown up in a variety of films including reprising her tough-gal role in The Matrix films, then appeared opposite Chris Rock in I Think I Love My Wife, and took part in other films as well. On a personal note, Torres had her first child with husband and Matrix co-star Laurence Fishburne in 2007.
Alan Tudyk - Hoban "Wash" Washburne
Alan Tudyk has been busy with films and plenty more since Firefly, appearing as Steve the Pirate in Dodgeball: A True Underdog Story, plus taking part in I, Robot, Knocked Up, Death at a Funeral, and 3:10 to Yuma.
Jewel Staite - Kaylee Frye
Jewel Staite is currently playing Dr. Jennifer Keller on Stargate Atlantis (which according to Wikipedia makes her the third Firefly cast member to appear in a Stargate series). She also starred in the brief Fox series Wonderfalls in 2004.
Summer Glau - River Tam
Summer Glau has become one of the most successful Firefly alums with her role on the so far successful Terminator: The Sarah Connor Chronicles. She also had short runs on The 4400 and The Unit, and will soon make an appearance as herself on The Big Bang Theory.
Sean Maher - Dr. Simon Tam
Morena Baccarin - Inara Serra
Source: Wikipedia: Firefly