Genesis 1:26
26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
Genesis 3:22
22 And the LORD God said, Behold, the man is become as one of us, to know good and evil: and now, lest he put forth his hand, and take also of the tree of life, and eat, and live for ever:
Genesis 11:7
7 Go to, let us go down, and there confound their language, that they may not understand one another's speech.
Isaiah 6:8
8 Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us? Then said I, Here am I; send me.
God speaks in terms of "us", which refers to the plurality of Persons within the Trinity.
While God could be addressing Himself using plural pronouns, it is more likely that He is speaking to heavenly beings that He has already created, and allows to be in His presence (see 1 Kings 22:19-22, Job 1:6, 2:1, Isaiah 6:1-3). But, notice the tenor of the argument from the Trinitarian perspective. In the Bible, God uses singular personal pronouns in reference to Himself, and is referred to using singular pronouns by others, thousands and thousands of times. Most Bible students can reflect on that fact, and notice that it is not the habit of God to contravene the plain message of those thousands of verses with 3 or 4 "hints" in which He reveals His "true nature" of being multi-personal. It is not a good hermeneutical principal to attempt to interpret thousands of verses in light of a contested understanding of 3 or 4 verses, which maintains something not actually explicitly stated in any of the passages - that God is multi-personal.
In the Old Testament, there would be no reason for God to "conceal" that He is multi-personal as a general rule, while leaving, for some reason, 3 or 4 hints that He is. Additionally, in the New Testament, there would be no reason to conceal that He is multi-personal, and yet, not even a single example of this is appealed to from the New Testament. These are good indicators that the Trinitarian reading of these texts is a severe over-reading, and that ideas are being imported which are not in the minds of the original authors.