So, the method is two-fold. First you must ALLOW free shipping to even exist in the first place in the system -> configuration. Second, you will create a custom “shopping cart price rule” to assign the free shipping to only the desired products.

Step 1

In your magento admin, first browse to system, then configuration.

Under sales, select the shipping methods category.

Open the Free Shipping tab. You only change two things.
For Enabled:  YES
For Minimum Order Amount, enter some crazy huge number. I entered 999999999. This is half of the trick. It enables free shipping as a “possibility” within the cart, yet no regular order would ever reach that level. If you already have this enabled (free shipping for orders over $50 etc..), then skip to step 2.

I’m not sure if it’s required, but I did the same thing under my standard shipping method as well. (for me it was UPS)

Free Shipping for Minimum Order Amount: Enable

Minimum Order Amount for Free Shipping: 999999999 (same crazy huge number as before)

Click the orange “Save Config” button to save your new settings.

Step 2

From the main Magento Admin menu at the top, go to Promotions, then select Shopping Cart Price Rules.

Click the Orange Add New Rule

Rule Information Tab:

Rule Name: Whatever You want
Description: Whatever You want
Status: Active
Customer Groups: As a default, Select all. (Hold the CTRL key and click)
Coupon Code. NOTHING (don’t enter anything here.)
Uses per coupon: 1000000000 (some huge number)
Uses per customer: 1000000000 (some huge number)
From Date / To Date: Leave blank to start now / end never.
Priority: 0 ?
Public in RSS Feed: no

Conditions Tab:

No changes needed, at least in my case.

Actions Tab:

Apply: Fixed amount discount for Whole Cart

Amount Discount: 0
Max. Quantity Discount Applied to: 0
Discount Qty Step: 0
Free Shipping: For Matching Items Only
Stop Further Rules Processing: NO

Apply the rule only to cart items matching the following conditions

This is key.
Click the green + button to add a new rule.
Select SKU.
Click “is” and change it to “contains“.
Click the “…“, enter the unique segment of the SKU on you free products. For example the SKUs for my gift certificates were gift25,gift50,gift75 etc.., so here, I entered “gift“. You may simply want to append your free product SKUs with the word “free” for clarity, then enter “free” back here. You can do this however you want, but be sure to not use anything that may be contained in a regular products SKU.
So for me that line read:
SKU contains gift

Click the Orange Save Rule Button.

That’s it! Try it out.
There’s another method I came across which is the same, except instead of anchoring the Shopping Cart Price Rule (promotion) to a products SKU, a custom attribute is created for that product and then the promotion is tied to that attribute instead of an SKU. See it here.