Besides Exodus 20:20 there are many other verses that view the worship of Yahweh from a henotheistic perspective.
And he said, "Tomorrow." Moses said, "Be it as you say, that you may know that there is no one like the LORD our God. (Exodus 8:10, RSV)
For this time I will send all my plagues upon your heart, and upon your servants and your people, that you may know that there is none like me in all the earth. (Exodus 9:14, RSV)
There is none holy like the LORD, there is none besides thee; there is no rock like our God. (1 Samuel 2:2, RSV)
And said, "O LORD, God of Israel, there is no God like thee, in heaven above or on earth beneath, keeping covenant and showing steadfast love to thy servants who walk before thee with all their heart. (1 Kings 8:23 and 2 Chronicles 6:14, RSV)
There is none like thee, O LORD; thou art great, and thy name is great in might. (Jeremiah 10:6, RSV)
Thou hast multiplied, O LORD my God, thy wondrous deeds and thy thoughts toward us; none can compare with thee! Were I to proclaim and tell of them, they would be more than can be numbered. (Psalm 40:5, RSV)
There is none like thee among the gods, O Lord, nor are there any works like thine. (Psalm 86:8, RSV) 
Probably the most quoted verse to prove that there is only one God is the following.
Hear, O Israel: The LORD our God is one LORD. (Deuteronomy 6:4, KJV & RSV)
Notice that this verse never says there is only one Elohim, but rather only one "Yahweh."
 I came across a website (Evidence for God from Science - http://www.godandscience.org/) with the article "Biblical Evidence that there is one God." The article provided 37 verses from the Bible claiming that there is only one God. Interestingly, not one of these verses make the claim "there is only one God," There were a multiple of verses that said that there was none like Yahweh, but this simply means that Yahweh is unique, not the one and only God. I also found it interesting that one of the verses was Psalm 86:8 which clearly states that Yahweh is among the gods.