We'd have to call BS on the En Vogue / Salt 'N' Pepa one. That's clearly a cover and we all know it.
However, it's very interesting to see how Sophie Ellis Bextor, Pitbull and even Eminem pretty much lifted straight from other songs and repurposed them for their own songs.
Sampling, as we all know, is a common part of modern music production and all of this is above board. The original artists - or whoever owns the rights - gets a cut and, for the most part, they're more than happy to have their songs heard again by new audiences.