The concept of free will is a concept that is Scriptural. I believe it is most often misdefined, as evidenced by Helen's definition.
Free will, theologically, is not the power of contrary choice, but the power to choose in accordance with the nature. God is a free being; he is not bound by anything outside himself. Having said that, he is bound by himself. Therefore he cannot do anything at all (e.g., he cannot sin). Yet his inability to sin does not mean that he is not free. He is free and able to do all things that are proper objects of his power.
In the same way, man created in the image of God, is free ... free to do all that is in accordance with his nature. He can make any choice he wants to, including the choice to serve God if he so desires. Natural man, because of depravity which affects his will, will not seek God (Rom 3:10-12) and cannot please God (Rom 8:5-8). He does not even desire to do so. Thus, he is not kept out of salvation against his will but rather because his own free choice is to reject. It is in accordance with his depraved nature. He desires to reject the biblical commands of God and in so doing is held responsible for them.
On the other hand, God implants in some a new nature, leading them to certain belief because their new nature has spiritual understanding (2 Cor 2:14; Eph 4:17ff; Eph 2:1-10; etc.).
In summation, "free will" is not explicated in Scripture. We are left to put together the teachings of Scripture about the freeness of God (who can't do everything) and the fact that man, like God, is bound by his nature. When viewed against the backdrop of Scripture, I think free will becomes a pretty easy subject. But then that's just me ...