As mentioned, Lady Gaga is amazing! A lot of her non-radio songs are great. I'm not a fan of Poker Face or the disco stick song (Love Game?), but she has some sick beats for working out.

I love Britney, too, but I'm a fan so I'm completely biased. Some old school Madonna is good too: Just try not to dance to Vogue, I dare you! I really like The TingTings as well.
I'm also a fan of the "girl power" workout, where I listen to angry girl music. Kelly Clarkson is great for this! hahaa. Also The Veronicas and Avril Lavigne (sometimes). Some of the songs by the Glee cast are very danceable, so if you're into the show, those might be a good option....
Oh! You mentioned the BEPs, and Fergie's solo album is AMAZING with tons of up-beat, girly songs
I don't know! There's just so much music in the world.