Not sure if it's more well known, but Joe Cocker's With a Little Help From My Friends is the better version (imho)
More well known, Relight My Fire, original by Dan Hartman

Red Sky Coven -> Wonderful World

I'll have to look that one up.

Hands up who else knows who Red Sky Coven are

For me the two best covers are Hendrix's "All Along The Watchtower" (already mentioned) and The Lemonheads fantastic "Mrs Robinson".

I quite like this one: http://www.youtube.com/watch?v=6zAewmM9zb4&NR=1

and this one too: http://www.youtube.com/watch?v=-dQV_8hhKZg&feature=related

"audiophile – Member
Fuel My Fire – The Prodigy, original by L7
Little Red Rooster – The Rolling Stones, original by Howling Wolf"

I think Willie Dixon might have the original claim to Little red Rooster

Nina Simone's version of Here Comes The Sun is arguably better known than George Harrisons version.

Jay-Zee cover of Ice T's 99 problems