As rehearsed as this bit is, it's still impressive to see Christina Aguilera absolutely nail impressions of Cher and Britney Spears (and to a lesser extent, Shakira). She performs "This Little Piggy" as Spears, though it's directly to the tune of "...Baby One More Time."
The impression of Cher performing the Folgers coffee theme is also nearly perfect, and Fallon competes briefly with a heightened Michael McDonald.
(Side note: The What's Trending office has spent the past 20 minutes debating the merits of Michael McDonald and The Doobie Brothers as a whole. I've just never been able to get into it.)