DONNA SUMMER & BARBRA STREISAND ''No More Tears'' 1979
The first minute's as slow and schmaltzy as a B-grade Barbra solo number — but then the song bursts into an ecstatic disco anthem.

REBA McENTIRE & LINDA DAVIS ''Does He Love You'' 1993
A woman (McEntire) has a frank dialogue with her husband's mistress in this weepy country-pop ballad.

BRANDY & MONICA ''The Boy Is Mine'' 1998
Crystal-clear harmonies, swirling Philly-soul strings, and a rumored real-life rivalry between the teen singers made this a megahit.

EVE & GWEN STEFANI ''Let Me Blow Ya Mind'' 2001
The rapper and the poppy No Doubt star complement one another's styles surprisingly well in this breezy hip-hop cut.

BRITNEY SPEARS & MADONNA ''Me Against the Music'' 2003
The combined efforts of two generations of MTV royalty aren't nearly enough to save this overproduced club mess.

BEYONCÉ & SHAKIRA ''Beautiful Liar'' 2007
Melding their unique voices over this Middle Eastern-tinged track, the ladies form an alliance against yet another two-timing guy. How hypnotic is it when they chant their own names?!