Mark Murphy has been writing really interesting posts in Android guys about the different business models around Android platform. We have chosen the most relevant ones adding some additional information:

• Build the app and sell it via the Android Market
• Give a free app, supported by ads (Adsense mobile, Admob, etc.)
• Give a free app, sell the customizations: plugins, analytics, additional features, account (e.g. RTM, Spotify), etc.
• Sell development tools: GUI toolkits, portability tool, …
• Develop custom applications for others (e.g. Droiders)
• Became a trainer of Android development
• Specialize in porting iPhone, Windows Mobile, J2ME applications to Android.
• Promote your ability to port some critical parts of the apps to the Android Native Development Kit (NDK)
• Became an specialist building cross-platform apps using HTML, Javascript and CSS (e.g. PhoneGap, Titanium, Rhodes)
• Became an expert to build apps with GPS and backend capabilities of HTML 5.
• Build an App Generator (e.g. ePUB, RSS, audiobooks) selling this generated apps.
• Became a specialist building Augmented Reality Layers for Layar or Wikitude

Mark has done an incredible job posting these business models, you can read the different post for more information (Part 1, 2, 3, 4, 5 and 6)