I think this one is important to point out as not only is it one of the 10, other verses later on define more what God is talking about when He makes this statement. I have heard several sermons on us putting people or possessions or other things before God, thus becoming our god, and I agree with that. There is also another aspect to this as well though. God has a certain way He wants to be worshipped. The golden calf incident shows this. Many read it and say “Oh, they made a false god and were bowing down to it, that is what made God angry.” However, if you read what Aaron said about the object once it was created, they made it FOR God, they were going to have a celebration to Him the next morning. It was part of their worship to Him, because that is how they learned to worship in Egypt. This is not how God wants to be worshipped, so He got angry about it.
We all need to take a look at our lives and our worship of Him and compare it to what He says He wants, not what our hearts desire.