I just got finished with a website project (https://www.earpops.com). My client needed to offer a free item for every 3 purchased (of the same item). Shipping would still be charged for each free item. That Software Guy's add-ins (3 of them) worked like a charm.
I used Combination Discounts to set up the discounts, which is very flexible. I then used the Preview add-in to show the discount in the cart to visitors, and Checkout Candy to display a message in the shopping cart to advertise the offer and to let customers know when they qualified for the discount.
The way I set it up was that I made an identical 'promotional' item to be offered as the free item. I gave it the full price so that if visitors happened to stumble across it, they could not just add it and get it for free. That way, you can't cheat the system. Combination Discounts applies a 100% discount to the promotional item only when the set number of the non-promotional item has been added to the cart.
I also used the hide categories add-in (in downloads section) to hide the promotional item and the category it's in from prying eyes.
That Software Guy's add-ins were a breeze to install and use if you read the readme files provided. I had the whole thing set up in about 2 hours, and I'm new to Zen Cart. Scott was very quick getting back w/ me, and I had the site up the next day.