Felice and Boudleaux Bryant wrote a ton of songs more succesful for the cover artist than for them;
http://en.wikipedia.org/wiki/Boudleaux_ … ice_Bryant
Although he was a lyricist, he may fall under the catagory you're looking for;
http://en.wikipedia.org/wiki/Bernie_Taupin
Ditto for this guy;Robert Hunter (Grateful Dead)
http://en.wikipedia.org/wiki/Robert_Hunter_(lyricist)
And last but not least, this guy might take the cake. Ladies and gents, LEADBELLY;
http://en.wikipedia.org/wiki/Leadbelly
"Lead Belly has also been covered by Ry Cooder, Lonnie Donegan, Grateful Dead, Johnny Cash, Gene Autry, The Beach Boys, Odetta, Led Zeppelin, Billy Childish (who named his son Huddie), Mungo Jerry, Nirvana, Paul King, Michelle Shocked, Tom Waits, Ron Sexsmith, British Sea Power, Rod Stewart, Ernest Tubb, Nick Cave and the Bad Seeds, The White Stripes, The Fall, The Doors, Smog, Old Crow Medicine Show, Spiderbait, and Raffi, among many others."