The Detective Fiction on Stamps website says Sherlock Holmes is a popular topic on stamps. In fact, they proclaim him "the champion," with a page of his own.
According to the site, "Over a dozen countries have produced Sherlock Holmes-related postal offerings, beginning in 1972 with Nicaragua's 50th Anniversary of Interpol issue,'The Twelve Most Famous Fictional Detectives,' in which Holmes was on the high value, and continuing through the 2009 Monaco issue commemorating the 150th anniversary of the birth of Sir Arthur Conan Doyle."
Shown above, 2009 release from Alderney (Guernsey) marking the 150th anniversary of Holmes creator, Sir Arthur Conan Doyle.
To see more stamps featuring the crime solver extraordinaire, click here.