I can only think of a few which he has either covered or quoted in interview and stuff.

Amy Winehouse - Love is A Losing Game
Sade - Love is stronger than Pride.
The Temptations - Just My Imagination
Radiohead - Creep

Suppose you lot know loads more??

He mentions a lot in "Musicology". He's also mentioned Chante Moore in "Ripopgodazippa".

Either live or on record, he's covered The Stylistics, Joni Mitchell, Joan Osbourne, The Delfonics, Sheryl Crow, Sly, James Brown, Santana, Graham Central Station, B-52's, The Ohio Players, Earth Wind & Fire, The JB's, Salt N' Pepa, Rufus & Chaka Khan, and a host of others.