The players wearing #21 at the time being are Jason Johnson (he wore #21 in Detroit, but it's retired in Cleveland for Bob Lemon, he is listed as #16 on the Indians website), Choo Freeman, Chad Moeller, Mark Kotsay, Jon Lieber, Jason Ellison, and Jason Marquis. I was using the ESPN.com rosters because I could through them more quickly, but numbers are all subject to change as rosters change through the spring.
Interestingly, no Latino players are wearing #21 to honor Clemente now that I can find. In addition to Pittsburgh for Clemente and Cleveland for Lemon, the Braves have also retired #21 in honor of Warren Spahn. Arky Vaughan also wore #21 when he played in Pittsburgh.