Madonna, Grace Jones, David Bowie, Bruce Springsteen, and Elton John come to mind first. Those are all in-your-face inspirations, but she definitely has other subtle inspirations.
Björk
Grace Jones
Madonna
Britney
David Bowie
Freddie Mercury
Andy Warhol
Marilyn Monroe
Gwen Stefani
Michael Jackson
Janet Jackson
Elton John
Whitney Houston
Bruce Springsteen
and probably some others