Everybody loves free apps. We definitely don't mind paying for premium, high-quality applications (the developers deserve it!) but there are few things that beat a wide selection of quality, free applications. Luckily, Android has free apps in droves. According to Distimo, Android Market has the highest concentration of free applications with respect to the total amount of applications available. Basically with 57% of applications being free, Android Market has the highest percentage of free applications of any app store.
To be fair, Android Market only allows paid applications in 11 countries (compared to the iPhone App Store's 77) so there's a bit of uneven playing field going here but compared to the rest of the app stores, Android Market is far and away the leader in free applications. Add that to the thousands of reasons why we love Android.