mysql: Show GRANTs for all users
select * from information_schema.user_privileges;


EDIT:

As mentioned by Shlomi Noach:


  It does not list database-specific, table-specific, column-specific,
  routine-specific privileges. Therefore, the grant GRANT SELECT ON
  mydb.* TO myuser@localhost does not show in
  information_schema.user_privileges. The common_schema solution
  presented above aggregates the data from user_privileges and other tables
  to give you the full picture.