If you can't afford to pay for an advertising campaign for your app, you should look at the following sites that allow you to trade install, links and banners with other developers. You promote them, and they promote you for free. The process is fully automated and fair. Keep in mind that when you use this method to increase your mobile app installs, you are using advertising space that you could have used to display third party ads that would bring you cash.
Adtwirl.com - Performance-based, auction-style mobile ad network.
Mainly targeting the markets of Russia and Ukraine.
Use AdTwirl is also a link/banner exchange network in 1:1 exchange ration for free.
Tapdaq.com - Trade installs with other developers and cross promote your own app network.
