Auto Add Promo Items - professional magento extension
http://amasty.com/auto-add-promo-items.html

Automatically add free promo products to the cart. Make little surprising presents and customers won't hesitate about returning back to your store!
- Automatically add free promo items to cart
- Unlimited number of rules per shopping cart
- Works both with and without coupon code
- Clear indication of free items in the cart
- Classic 'Buy 1, get 1 Free' promotion