Although I suppose it is technically wrong to capitalize the word free in this sense, it is common practice in informal writing. What you are seeing is an emphasis being placed on the word "free" in order to draw your attention to it.
Although technically invalid, there is an increasing trend towards capitalization of words that are to be emphasised or "shouted", for example, the following is ungrammatical with its use of capitals, but uses artistic licence to give a better impression of the speaker's tonality:
Where did I put my car keys? Are they in here? No? Ok maybe here? No? Oh my goodness I need my keys. In here? No. Oh My God. Where are my keys? On the shelf? No. In the draws? No. On the fridge? Seriously. OH MY GOD. Where are my FREAKING keys!? Oh, here they are. Thank GOD.
In this case we can see a lot of abuse of grammar going on. In particular, excessive capitalisation of
Oh My God
suggests an increasing frustration in the speaker's voice. Later in the sentence this is repeated again, but with more emphasis:
OH MY GOD
This means the same as "Oh my god", but in this case, with the implication that the speaker is now quite loud (and presumably angry).
Later in the sentence, the word FREAKING is capitalized as an emphasiser to show that the speaker is stressing that word, and there's also the completely ungrammatical !? mark giving additional emphasis and force to the question.
Note that the final GOD is emphatic as a sigh of relief, rather than stress.
Be aware that generally you should avoid this style of writing in business and formal settings.
You'll tend to find this style of emphasis more common on the Internet, where forums and instant messaging tend to be quite informal and "chatty".
You'll also see it in adverts, where the intention of the author is to grab your attention rather than to be grammatical, and in non-fiction writing as emphasis of spoken text.