Hey, Try putting ads in the application if it's that popular. Otherwise maybe if it's good enough turn it into a paid application business instead. ~ Slutz ~
Advertising, other companies' paid products, or advertising your own paid products. A free app that is widely used can make you known, and improve the sales of your other products significantly. You can also add paid features to your free apps if you can come up with something that won't chase your users away.
There are a number of different was to ammortize free software including: 1. Imbedding ads in the software 2. Using Google AdSense on the page 3. Upselling related products 4. Having paid support options 5. Building a large customer base then selling the product
If it's got a good success rate then consider having a better paid one programmed for you. Otherwise if you want to stay on the free zone then just embed adsense ads and the like on your app.