As far as we can ascertain, at least 40, although we have no doubt that there are more out there that we just haven't come across yet.
The following list, presented chronologically by release date, encompasses everything from German post-punk versions to English country covers, French rockabilly to Chilean nihilism. And a lot of other genres in between.
For each of those we know of, we've listed year of release and the album you can find it on.
Elvis Presley, Viva Las Vegas/Roustabout, 1964 -- the original
Dead Kennedys, Fresh Fruit for Rotting Vegetables, 1980
Stephen Cummings, Lovetown, 1988
Nina Hagen, Punk Wedding, 1988
Residents, The King and Eye, 1989
Bruce Springsteen, Last Temptation of Elvis, 1990
The Special Consensus, Hey, Y'All, 1990
ZZ Top, Greatest Hits, 1992
Larry Victor, Our Time Will Soon Go , 1993
Dread Zeppelin, No Quarter Pounder,1995
Shawn Colvin, Till the Night is Gone, 1995
Blues Brothers & Friends, Live From the House of Blues, 1997
Wayne Newton, Viva Las Vegas!, 1997
Hell City Hell Philharmonic, Hell City Hell, 1998
Gipsy Vagabonds, Vive La Vide, 1998
Mr. Pink, Red White and Pink, 1998
Lynette Morgan & Her Tennessee Rhythm Riders, Little Red Wagon, 1999
The Legendary Raw Deal, Southern Boys, 1999
Hannah Elisabeth, The Sixties Show, 1999
Nine Inch Elvis, Nine Inch Elvis, 2000
Ann Margrock, i.e., Ann-Margret, The Flintstones in Viva Rock Vegas, 2000
Billy Swan, Like Elvis Used to Do, 2000
Jailbirds, Viva Las Vegas, 2000
Swing Cats, A Special Tribute to Elvis, 2000
Venetian Snares + Speedranch, Making Orange Things, 2001
Fiskales Ad-Hok, ("Viva Santiago,") Calavera, 2001
Great Pretenders, Point of View, 2002
Speed Chicken, Der Mann Zum Hochbeamen (import), 2002
Paul Ansel's Number Nine, Sweet Inspirations, 2002
Hollywood Sound Orchestra, Elvis at the Movies, 2002
Phil Cody, Mad Dog Sessions, 2002
Englebert Humperdinck, Viva Las Vegas (import), 2002
King Junior, Tribute to the King, 2003
Lee Rocker, Burnin' Love, The Best of Lee Rocker (import), 2004
Lisa Dames, If These Walls Could Talk, 2004
Royal Crown Review, Greetings From Hollywood, 2004
The Grascals with Dolly Parton, The Grascals, 2004
The Thrills (Featuring James Burton), The Irish Keep Gate-Crashing, 2004
Washington Dead Cats, Treat Me Bad, 2005
Cornell Hurd Band, Texas By Night, 2006