There are a couple of free ones, but they are rubbish and usually require a third-party dll to be included in your distribution. We use a non-free third party tool called Envision, but we have since modified it so much that it is virtually a whole new system.
Before you spend too much time and effort writing your own TWAIN integration check out our Quickscan (http://www.burrotech.com/quickscan.php) – it is a low cost command line scanning utility that supports jpeg, bmp, tif and pdf output as well as allowing you to set resolution, adf, duplex etc.
Paul
Burrotech