As the late afternoon sun beat down over the grounds of Bonnaroo 2009, Karen O of the Yeah Yeah Yeahs was prowling across Which Stage, sporting wild yellow-and-black leopard-print tights and a billowing tank top, when she suddenly stopped for a second, panted into her microphone and shouted: “It’s hot out there, Bonnaroo!” Over at That Tent, futuristic pop singer Santigold echoed a similar sentiment: “Whooooooo!” she hollered. “I’m dripping with sweat but I’m having a really great time!” (Click above to see footage from the Yeahs’ set and watch their pre-show limber up, plus check out our interview with French rockers Phoenix with a dash of their performance of “Lisztomania.”)

(Dive into Bonnaroo ‘09 in our gallery of shots from the stage and beyond.)

It was brutally hot and humid for the second day of Bonnaroo — the temp soared to almost 90 degrees — and there was little shade for fest-goers to escape the swelter. But the afternoon’s performers still managed to bring high-energy performances and attendees were willing to sweat it out (or, in some cases, pass out from exhaustion on the lawn) to catch some great sets, which were heavy on indie-rock bands for the first part of the day. Animal Collective dug deep for some trippy, electronic grooves; Dirty Projectors were joined by David Byrne for a set of knotty, Afro-pop-inspired art-rock and Yeah Yeah Yeahs got the crowd pogoing to raging versions of “Date With the Night” and their fantastic new single “Zero,” as TV on the Radio’s Tunde Adebimpe rooted them on.

Later that night, the Beastie Boys delivered a set of classic jams (”Intergalactic”; a raging, set-closing “Sabotage”) and a couple of deep cuts (”Heart Attack Man”). They also unveiled a few surprises: the trio were joined by Nas for a new jam off of their forthcoming album Hot Sauce Committee. Over a minimal, dubby groove and squealing turntable scratches, the guys prowled all over the stage, trading funny boasts (”I’m Mike D, the man of mystery… excuse my competition while I sharpen my blades”) and tossing out references to Wolf Blitzer, Splenda and Stax Records. A beaming Nas looked like he was having a blast. “Too many rappers and their still ain’t enough MCs,” they shouted. Afterwards, the trio launched into “Paul Revere,” which Yauch said was personally requested by Nas. (Just before heading off stage, Yauch gave shout outs to bands like TV On the Radio, Santigold, Nas and Yeah Yeah Yeahs before neatly summing up a theme of Bonnaroo Day Two: “Who rocks the house tonight? New York City!” New York bands were out in full force yesterday, including Grizzly Bear, Public Enemy, Dirty Projectors, David Byrne and Animal Collective.)

Of course, a majority of fest-goers were there to see Phish, who hit the stage around 11 p.m. for a non-stop three-hour set. (Unlike traditional Phish shows, there was no set break, making this one of the longest sets they performed since their marathon eight-hour set at their legendary Florida New Year’s gig in 1999. It was also one of the few times the group performed at a festival alongside other acts.) The jam kings proved why they are one of the few bands who can dominate a crowd as large as the one at Bonnaroo: they turned out a terrific set of fan-favorites (”Chalkdust Torture,” “Possum”), new tunes (”Stealing Time From the Faulty Plan,” “Kill Devil Falls”) and epic, stretched-out jams (the super-funky “Wolfman’s Brother”; the twisted, shape-shifting “Divided Sky”). There were also genuine moments of audience-fan synergy: during a mellifluous portion of their slow-building anthem “Harry Hood,” fans tossed around a seemingly endless supply of glowsticks, creating a pretty stunning visual. Guitarist Trey Anastasio seemed touched by fans reaction to Phish’s set: “I’m so incredibly happy to be here,” he said. “This is fantastic!”

Once Phish wrapped up around 2 a.m., the party raged on with late-night dance parties from Public Enemy, Femi Kuti and Positive Force, Paul Oakenfold and more. But nothing topped Girl Talk, who delivered a set of expertly mashed-up pop music (Rick Springfield’s “Jessie’s Girl” and Kelly Clarkson’s “Since You Been Gone”) atop propulsive house beats. The crowd was clearly psyched: fans leaped over the barriers to join Girl Talk’s Gregg Gillis on stage for an anarchic celebration. At the end, Gillis stood atop his DJ rig to thank fans: “A couple of years ago, I would maybe play for 20 minutes, but tonight I played for an hour-and-a-fuckin’-half! Fuck yeah!”

(Click to transport yourself into Girl Talk’s dance party with footage from Gillis’ ‘Roo set, and catch our interview with Gillis in our second Girl Talk video, below.)