select min row in a group

This is really an SQL question, not specific to PostgreSQL. I have a table:

ValTable
======
id    int
ext_id    int
value    float

For each ext_id (which references something in another table), I want to
find the row with the minimum value. The best I can come up with is:

SELECT * FROM ValTable AS v
WHERE v.value = (
    SELECT DISTINCT min(value) FROM ValTable
    WHERE ext_id = v.ext_id
    )

I feel like there has to be a way to do it without a nested query, probably
using GROUP BY. Any thoughts?

--Greg