I've seen the custom quoted in many siddurim to recite Psalm 130 (שיר המעלות ממעמקים), with the Ark open, between ישתבח and ברכו during עשרת ימי תשובה.
Why this Psalm? What special relevance does it have to עשרת ימי תשובה (Ten Days of Repentance between Rosh Hashanah and Yom Kippur)?