Correct way to select from two tables in SQL Server with no common field to join on

You can (should) use CROSS JOIN. Following query will be equivalent to yours:

SELECT 
   table1.columnA
 , table2.columnA
FROM table1 
CROSS JOIN table2
WHERE table1.columnA = 'Some value'

or you can even use INNER JOIN with some always true conditon:

FROM table1 
INNER JOIN table2 ON 1=1