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Michael Morgan
The above answers are spot on. I have found that the parables often have multiple lessons woven into them. When I read or hear this parable what speaks to me is that the labored are not happy with what the owner gave them or what the later laborers received. This to me represents how we often become self righteous in what rewards we think we deserve and what others deserve. Christ taught that the Father does not wish that any should perish, yet we often wonder just who will get the reward of Heaven.
In our status driven society today, even we the saved sometimes frown on God appearing to give as much or more to those we feel have not been "good enough ". The scriptures tell us that God allows the rain to fall on the righteous and the unrighteous.
In the end one lesson for me in this parable is to not only be happy with what God gives you, but we should also find joy in what He gives others. As sinner's saved by Grace, why would we ever question the Grace God gives someone else? The thief on the cross received the same gift of Heaven as the great men and women of God of times past. The greatest gift is eternal life with God, and I would not deny that to anyone.