1. Tori Amos covers Nirvana's Smells Like Teen Spirit
Tori Amos does a lot of soulsy covers of odd source material, including heavy metal and death metal originals (e.g. Raining Blood by Slayer).
2. Einat & Hakim cover The Black Eyed Peas' I Got a Feeling
These guys are pretty funky Israelis.
3. Alanis Morissette covers The Black Eyed Peas' My Humps
A joke, but shows that Alanis can't help sounding good even when she's not trying.
4. Jonathan Coulton covers Sir Mix-a-Lot's Baby Got Back
Also a joke, I suppose, but it's hard to listen to this and think that he's not singing something profound.
5. Pomplamoose covers Beyonce's Single Ladies
Pomplamoose has a number of odd, weird, funky covers, including an awesome cover of Lady Gaga's Telephone (which itself has an entire bevy of weird covers, from country rock to classical).
6. Sarah Bareilles covers Beyonce's Single Ladies
Another cover of Single Ladies, this demonstrates Sarah's talents, which go far beyond simple pop and ballad numbers.
7. Karmin covers Chris Brown's Look at Me Now
The most viewed cover on Youtube (56 million views and counting), the cover is funny, sweet, and silly; the original song is none of these. It's dumb, racist, sexist, stupid, and talentless. Karmin does a number of other interesting covers.
8. Mat Weddle covers Outkast's Hey Ya
Mat is aka Obadiah Parker.
9. Butch Walker covers Taylor Swift's You Belong With Me
Taylor twitted about how much she liked this cover and invited Butch to integrate his sound into one of her performances. Skip to about 2:40 if you don't want to see Butch peeing.
10. Greyson Chance covers Lady Gaga's Paparazzi
There are lots of kids and amateur wannabees covering pop songs on Youtube, but few can transform a recognizable smash pop hit and - astoundingly - make it better. And he's only 13, for crying out loud.
Birdy covers Bon Iver's Skinny Love
Bon Ivers is relatively unknown, and the original song is folk music. But it's still amazing. And she's only 15 years old.