"Free" is arguably the most potent word in marketing. It's also one of the more popular search terms on Google. Here are results for a ridiculously diverse collection of search terms (number of Google results are in parenthesis).
10. "Beatles" (89,900,000) 9. "Britney" (141,000,000) 8. "Obama" (191,000,000) 7. "Jesus" (259,000,000) 6. "Football" (550,000,000) 5. "Sex" (869,000,000) 4. "Family" (1,710,000,000) 3. "Love" (2,380,000,000) 2. "Google" (2,700,000,000) 1. "Free" (4,530,000,000)
Actually, one marketing term is even more popular than "free" (see the visual). Check out other interesting search terms on Google and add the results to "Comments."