Summer in the city really means celebrities in the city. Seen on the scene, in the last week alone:

Thursday, June 7
Perhaps the most celeb-studded party Chicago has seen in recent years, the official Ocean’s Thirteen opening bash took place at Jerry Kleiner’s Room 21 last Thursday, following the movie’s premiere at AMC River East 21. Hosted by Vanity Fair, the party doubled as a charity event for Darfur; the cheapest ticket went for a cool $2,000. “Everything was draped in red, and it looked like the VIP-VIP-VIP room of Las Vegas’s best casino,” says guest Jonathan Landan, president of the marketing company A-Life Group. “George Clooney, Matt Damon, Don Cheadle, Bernie Mac, and Ellen Barkin all hobnobbed … and took pictures with the people that approached them.”

Friday, June 8

Jeremy Piven and Billy Dec at the Piven Theatre Workshop Benefit at Rockit Bar and Grill

Last Friday, after hosting the Generation Obama event at Union Station, Billy Dec and Emmy winner/Entourage star Jeremy Piven hung out at The Underground. The night’s highlight came when Dec and Piven jumped up on the couches (shades of Tom Cruise, anyone?), hyping the crowd for their annual Piven Theatre Workshop charity event the following night at Rockit. Also in the crowd was British hip-hop sensation Lady Sovereign, fresh off opening for Gwen Stefani and Akon at the First Midwest Bank Amphitheatre in Tinley Park.

Speaking of Akon, the hip-hop and R&B singer put in an appearance at Enclave Friday, following his concert with Stefani. Akon performed at Enclave for about 45 minutes; a spy tells me he was “super-friendly and interactive with fans.” (Here’s hoping there was no dirty dancing with a minor this time around.) At one point, Akon actually threw himself into the crowd. Hey, at least he didn’t throw someone else.

Saturday, June 9, and Sunday, June 10
As for the Piven Theatre Workshop Benefit, the Saturday event, held at Rockit, raised $150,000 with help from a live auction; items included a walk-on role in Piven’s hit HBO show, Entourage. Afterward, it was back to The Underground, where Piven held court at Dec’s booth up front. Frisky bachelor Piven doled out smooches to a crowd of attendant ladies with cameras.

Pete Wentz and Ashlee Simpson at Debonair Social Club

Also Saturday: Wilmette native Pete Wentz, Fall Out Boy bassist and back-up vocalist, played DJ at Enclave, at the official after-party for the band’s concert at Northerly Island. Wentz’s main squeeze, Ashlee Simpson, sister of Jessica, spent the evening jumping up and down while Wentz spun-that is, when the couple wasn’t acting all lovey-dovey during Wentz’s turntable breaks. “They could not keep their hands off each other!” a witness says. “They acted like high schoolers in puppy love.” Simpson’s drink of choice? Johnny Walker. In regards to her sister’s on-again, off-again relationship with John Mayer, Ashlee said only, “She’s doing great.” On Sunday, the couple headed to Debonair Social Club for more canoodling.

And over at Reserve, former blink-182 drummer Travis Barker showed up Saturday night with his stunt-bike crew, Team Famous, and buddy/rapper Paul Wall. On Sunday, Barker checked out the Reserve folks’ newest nightspot, Crescendo (222 W. Ontario St.), in River North. He arrived early and stayed late, and his entourage “sat in the middle of all of our regular clients, just partying,” Reserve rep Anderson Bell says.

Tuesday, June 12
Mandy Moore-star of the upcoming film License to Wed, out July 3rd-performed at Park West Tuesday in support of her new album, Wild Hope, which drops June 19th. While in town, Moore dined at Rockit twice: for dinner Monday and lunch Tuesday. “She really liked the turkey burger and sweet potato fries,” a souce tells me. I mean, who doesn’t?

Want to play your own game of celebrity spotting? Here are two opportunities:

• Thursday, June 14: Rob Base and DJ E-Z Rock perform live at Cabaret. Expect hits such as “It Takes Two” and “Joy and Pain.” Doors open at 9 p.m.; arrive early to avoid lines. Cover is $10 for women and $20 for men.

• Friday, June 15: Good Charlotte frontman Joel Madden-a.k.a. Nicole Richie’s beau-performs a live DJ set at Crobar to promote the band’s new album, Revival. Tickets are $20; doors open at 10 p.m., with Madden scheduled to go on at 12:30 a.m. Maybe Richie will pull a Simpson and make an appearance as well.

Photography: (Image 1) José Angel; (Image 2) Ashley Klich