Batman gets a birthday bash! Days before news broke that Ben Affleck would be playing Batman opposite Henry Cavill's Superman in the Man of Steel sequel, the Oscar winner celebrated his 41st birthday with family and friends -- with wife Jennifer Garner coordinating the party at their home in L.A.'S Brentwood neighborhood on Sunday, Aug. 18.

"There were about 30 people who showed up around 2 p.m. for an afternoon party," an eyewitness tells Us Weekly. "There were kids running around in the yard having a blast." Adding, "It was a relaxed afternoon and people didn't bring gifts. They all started to leave after dinner around 6 or 7 p.m."

The intimate get together also included the Runner Runner star's BFF, Matt Damon, who attended with his wife Luciana and four daughters Isabella, 6, Gia, 4, Stella, 2, and Alexia, 14, Luciana's daughter from a previous relationship. The Elysium actor, 42, and his wife of nearly eight years, recently moved from New York to Los Angeles with the family.

The Good Will Hunting co-stars have been able to spend more time together since the move, and has also brought their wives closer. "Jen likes to throw parties at the house, she has a clique in LA that Luc is getting introduced to," another source adds.

For more details about Affleck's new role, watch this week's Hot Stuff video, as explained by Us Weekly entertainment director Ian Drew and special guest Bridgit Mendler, the up-and-coming actress and singer promoting her debut album, Hello, My Name Is.

Also in the roundup: Simon Cowell's pricey present to Lauren Silverman, the Downton Abbey Season 4 wrap party, and more!