Best songs played by non-author artists...

One of my favorites (have it on a 10" EP):

"Ode to Billie Joe" - Phranc

We regularly play the Zappa version of Whipping Post in our room at the audiophile shows. I think one of my all time favorites though is the version of Ballad of Easy Rider done by Fairport Convention that appears on one of my favorite recordings, Richard Thompson: Live (More or Less). The lead vocal is done by Sandy Denny and is absolutely ethereal.

O yes the Whipping Floss bt Zappa'classic.
Any song on the Styx Big Bang Theory.Demo quality sound.And their version of the Walsus by the Beatles is quite good.

The term you are looking for is "cover song." There is already a massive thread on this forum about the best rock covers.

Flaming Lips do Pink Floyd
http://www.youtube.com/watch?v=hHS1pi-y3Kc

Jeff Buckley does Van Morrison
http://www.youtube.com/watch?v=8kPUT_1d4dA

and Nirvana doing David Bowie
http://www.youtube.com/watch?v=89f9EPq_GtU

of course, Jeff Beck's A Day in the Life is the best of all time,
http://www.youtube.com/watch?v=CziRncMpmTI

I've got a million of 'em, but love all of these

I see Mateored, but I'm not talking about just rock.

On his latest, and sadly last album, Ghost on the Canvas (he has Alzheimer's) Glen Campbell does a great version of Hold on Hope, originally written by Guided by Voices. I think his version is far superior to the original.

Dylan by the Dead for sure!!!
Postcards of the Hanging

Alexwgoody,
Nirvana's "The Man Who Sold the World" is a memorable rendering for sure, as powerful as the original. I had never heard of the Jeff Beck version of A Day in the Life, so I followed your link. I found it boring and predictable.

Roxy:

I would have to say that the album version of Beck's couldn't touch how he played it live. He did it for (If I remember correctly) the Rock and Roll Hall of Fame and last time I saw him live. I guess my enjoyment of the album cut was colored by seeing it live and it being incredible, plus, I'm generally a big fan of his (He did win a grammy for it though, I believe)

Maybe you'll like him and Sting playing "People Get Ready"
http://www.youtube.com/watch?v=WOnz2056RGQ

While A Day in the Life is a great cover, Jeff Beck's cover of Over the Rainbox I saw him perform live was much better.

Cassandra Wilson - Come On In My Kitchen (Robert Johnson)
http://youtu.be/t4Tu2IiZ3to
Cassandra Wilson - The Last Train To Clarksville (Boyce & Hart)
http://youtu.be/faK8TxS-D1w
Cassandra Wilson - The Witichitaw Lineman (Glen Campbell)
http://youtu.be/lWwDgYlGeZA
Yeah, I like Cassandra WIlson!

I like many cover albums. Among my favorites:

"Odetta Sings Dylan" (amazingly good version of "Times They are a Changin'").

"If I Were A Carpenter" (various artists covering Richard and Karen Carpenter songs--check out "Yesterday Once More" done by Redd Kross and "Top of the World" by Shonen Knife).

"Nouvelle Vague" (self titled--covers of new wave classics in a bossa nova style. Check out their version of The Clash's "Guns of Brixton").

"Twelve" by Patti Smith (her cover versions of some of her favorite songs).

"Wrecking Ball" by Emmylou Harris

"Golden Throats 1" (great compilation of spectacularly bad vanity covers by celebrities--includes famous Shatner versions of "Lucy in the Sky" and "Mr. Tambourine Man," the recently deceased Andy Griffith doing "House of the Rising Sun," Sebastian Cabot reciting the lyrics to "It Ain't Me Babe," Mae West doing "Twist and Shout").

"Stay Awake" (covers of Disney movie classics--check out Tom Waits' "Heigh Ho" and Sinead O'Connors "Some Day My Prince Will Come").

Elton John - Pinball Wizard and Lucy in the Sky

Patty Smith - Because the Night (does anyone remember the Springsteen version?)

Shelby Lynne - Breakfast in Bed

Antony Hegarty of Antony and the Johnsons covers "If it be Your Will" by Leonard Cohen. Talk about pathos. His interpretation will leave you speechless. It's on a live concert album featuring multiple artists performing excellent takes on Cohen's songs. The concert movie is also excellent.