Hmmm. Now this is a good question.
I'm being totally serious here.
I don't remember when I first discovered the Free RPG resources the internet provides, but discover them I did, and I haven't regretted it one bit. I have a number of Free RPGs downloaded that I absolutely adore. Choosing a favorite is very difficult.
Among the best?
Doom and Cookies is a clever, simple RPG about children at an orphanage, and their desperate need to escape before doom comes to claim them. It's a bit of Oliver Twist, crossed with Lemony Snicket's, 'A Series of Unfortunate Events'. Definitely a great choice as a first RPG for both youngsters, and first time older players.
Risus. Ah, what can I say about this amazing game. Simple, sublime, brilliant. All true, yet an understatement to how great, and versatile this thing is. Created by the ever-so-inspiring S. John Ross, Risus is a generic RPG system with a distinctly fun feel, and it formed the basis of my own homebrewed Muppets RPG (though I eventually tweaked it to where I feel the Muppets game is kind of its own thing).
You know what...while it's on my mind...
Yeah! My own Jim Henson's The Muppets Role Playing Game! This completely unofficial, homebrewed RPG is so super fun that I am going to list it as a favorite free RPG. You think me egocentric? Well your opinion be damned! Play it, and then decide.
Can't wait to update this baby when the new ABC prime time series comes out.
There are many more Free RPGs I could list, including Ryuutama (The Dragon's Egg - The Unedited Translation Edition is free), Extended Mission (which I modified - heavily), and many more. I highly recommend checking out the websites 1KM1KT, John Kim's Free RPGs on the Web, and of course The Free RPG Blog by Rob Lang.
Generally speaking, independent gaming publishers aside, it seems that many creative game designers are giving away better, and more interesting games than the larger pay-for-product companies are trying to sell.