Download SAM - SlideME Application Manager is a client that allows users to discover, download and install applications to their device. With the upcoming full client, users will be able to interact with developers and to make comments or even to hire developers. But you don't have to wait for the full-client, you can get started now with SAMce, an open-source community edition.
For the current Android SDK android.permission.INSTALL_PACKAGE does not work for external apks. To get around this limitation, use the SlideME SDK Installer. This is an external process that receives Content URLs over a socket connection. It then fetches content from the received URL, saves it to the file-system and then installs it using adb. To run the process.
java -jar android-sdk-installer-1.0-CE.jar
The service runs on 8090, so you will need that port clear. You can also use this service with your own clients. Merely have your Android client do something similar to the following. It will download and install the content at the specified URI.