The term "Common Grace" is often thrown around in reformed circles. What is this concept? What is the scriptural basis for the belief?
The idea of Common Grace is that the grace of God is accessible and granted to all people (ie "common" to all people). This is different from "special" or "saving" grace in that special grace is extended only to the elect.
More clearly, it's the idea that there is grace available and inside everyone. It can be seen in such verses as:
9 “Which of you, if your son asks for bread, will give him a stone? 10 Or if he asks for a fish, will give him a snake?
This is common grace in that everyone (even pagans, idolaters, and abominations) still love their children and will try to take care of them. These parents extend grace even though they are not Christians.
However, even though this grace comes through these "evil" parents, it actually comes from God:
Every good and perfect gift is from above, coming down from the Father of the heavenly lights, who does not change like shifting shadows.
Because of this, God has given grace (and love and mercy) to all humans regardless of their state of salvation or their obedience to him.
This is just "common" grace.
Wikipedia has a lot more regarding it, but that's a simple definition of it and the biblical basis for it.