You can have an offline event and introduce your app to the media and first movers. You can participate in events like Techcrunch in San Francisco or we are preparing an event in November for technology companies to promote themselves. You can join Mobile Monday events and network with people to introduce your app. You can keep a blog and write about the usefulness of your app targeting SEO keywords. You can go on forums and search conversations on the web talking about your topic and leave comments linking to the landing page Alvin mentioned or to the download of the app. You can pay for app advertising--there are many programs. You can make videos talking about your app and post them on forums in facebook etc.