"For in Christ Jesus neither circumcision nor uncircumcision counts for anything, but only faith working through love." (Galatians 5:6 ESV)
So, for the Christian, what qualifies as a "good work"?
The Heidelberg Catechism summarizes this concisely...
90. Q. What is the coming to life of the new nature?
A. It is a heartfelt joy in God through Christ, and a love and delight to live according to the will of God in all good works.
 Ps. 51:8, 12; Is. 57:15; Rom. 5:1; 14:17.  Rom. 6:10, 11; Gal. 2:20.
91. Q. But what are good works?
A. Only those which are done out of true faith, in accordance with the law of God, and to His glory, and not those based on our own opinion or on precepts of men.
 John 15:5; Rom. 14:23; Heb. 11:6.  Lev. 18:4; I Sam. 15:22; Eph. 2:10.  I Cor. 10:31.  Deut. 12:32; Is. 29:13; Ezek. 20:18, 19; Matt. 15:7-9.
John Frame explains this as works according to a right standard (God's word), done with a right motive (faith, love), toward the right goal (glorifying God). [The Doctrine of the Christian Life, 2008]
Frame notes, however, these are not three criteria but really only one... God's will (right standard) is that we trust and love him (right motive) and manifest our faith and love in ways that reveal his glory (right goal). The right motive (faith working through love) seeks to please him according to his will (right standard) and for his glory (right goal). The goal of the glory of God will be seen in people trusting and loving him (right motive) enough to obey his will (right standard). See Dr. Frame's comments on this here.
Finally, here are three short verses that support this understanding of good works...
"For whatever does not proceed from faith is sin." (Romans 14:23b)
"Whoever has my commandments and keeps them, he it is who loves me." (John 14:21a ESV)
"So, whether you eat or drink, or whatever you do, do all to the glory of God." (1 Corinthians 10:31 ESV)