IN Clause with NULL or IS NULL

Your query fails due to operator precedence. AND binds before OR!
You need a pair of parentheses, which is not a matter of "clarity", but pure logic necessity.

SELECT *
FROM   tbl_name
WHERE  other_condition = bar
AND    another_condition = foo
AND   (id_field IN ('value1', 'value2', 'value3') OR id_field IS NULL);

The added parentheses prevent AND binding before OR. If there were no other WHERE conditions (no AND) you would not need additional parentheses. The accepted answer is misleading in this respect.

SELECT *
FROM tbl_name
WHERE coalesce(id_field,'unik_null_value') 
IN ('value1', 'value2', 'value3', 'unik_null_value')

So that you eliminate the null from the check. Given a null value in id_field, the coalesce function would instead of null return 'unik_null_value', and by adding 'unik_null_value to the IN-list, the query would return posts where id_field is value1-3 or null.

An in statement will be parsed identically to field=val1 or field=val2 or field=val3. Putting a null in there will boil down to field=null which won't work.

(Comment by Marc B)

I would do this for clairity

SELECT *
FROM tbl_name
WHERE 
(id_field IN ('value1', 'value2', 'value3') OR id_field IS NULL)