This shipping module is modified from Freecount.zip which allowed you to offer free shipping w/ a minimum number of products in the shopping cart.
This edition of the script offers free shipping with a minimum order amount in dollars (or whatever your local currency).
tar -xvzf freeamount.tar.gz
this will place a file in catalog/includes/modules/shipping/ and a file in catalog/includes/languages/english/modules/shipping/. Feel free to edit files to tailor to your situation.
Now load your administration pages and configure "Free Shipping w/ Minimum Amount". Send email if you have questions.