Several of the free sites have a revenue sharing scheme where you et about 1/2 of the earnings. Hubpages and Squidoo are a couple that come to mind, maybe Tumblr as well, in some cases if you get a custom domain name you may be able to keep all of the revenue.
Google's Adsense is the most popular passive revenue source for blogs, their min payout is a high $100 typically requiring tens of thousands of visitors to earn, some sites never reach that level.
Developing a site that gets any decent traffic and ad revenue takes a considerable investment, having a custom domain name allows you to relocate the site to a different host with a limited disruption of incoming links and search ranking it has acquired. Many free hosts have paid upgrades, wordpress.com the free host of the industry standard blog engine for example charges for business sites or custom domain names, Google's Blogger.com is one with no paid upgrades, allowing you to add a custom domain name obtained elsewhere (the process may be a bit more tricky than a paid host)