Once we attain heaven, we will be given a new nature, purged of our old sinful nature.
2 Peter 1:4 (NIV)
Through these he has given us his very great and precious promises, so that through them you may participate in the divine nature, having escaped the corruption in the world caused by evil desires.
Once we escape this corruption, we will gain the sinless state of Jesus.
1 John 3:5-6 (NIV)
5 But you know that he appeared so that he might take away our sins. And in him is no sin. 6 No one who lives in him keeps on sinning. No one who continues to sin has either seen him or known him
2 Corinthians 3:18 (NASB)
But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as from the Lord, the Spirit.
Now, while Jesus was free from sin, he also had free will:
John 10:17-18 (NIV)
17 The reason my Father loves me is that I lay down my life—only to take it up again. 18 No one takes it from me, but I lay it down of my own accord. I have authority to lay it down and authority to take it up again. This command I received from my Father.
So, there will be no sin in heaven. This isn't because we don't have free will, but rather because we simply won't sin. It won't be in our nature. Just like now, it's not in our nature to fly. So then, it won't be in our nature to sin.
However, there will be freedom in heaven:
2 Corinthians 3:17 (NASB)
Now the Lord is the Spirit, and where the Spirit of the Lord is, there is liberty.
Clearly, there is freedom in the Lord. We don't loose our freedom simply because we're in heaven.
One final note is a simple call to logic: How can heaven be paradise if we lose our freedom and free will?
One way to think about this is a world where sin isn't an option. Partially because it's not in our nature and partially because it's not available to us. Imagine, for example, life in the Garden of Eden, but without the snake.
There is no reason to sin. There is no need to sin. There is no desire to sin. Yet there is complete free will.
(Note that I give this as a simple way to reconcile this in our minds, not as an actual answer. Because in truth, free will allows for sin. But, in heaven there will not be any sin--per my answer above.)