The FDroid Repository is an easily-installable catalogue of FOSS applications for the Android platform. The server contains the details of multiple versions of each application, and the Android client makes it easy to browse, install them onto your device, and keep track of updates.
Open the FDroid application, press the menu key, and choose update. The lists should then be populated with the FOSS apps already in the repository. Stuff you already have will be automatically detected, and put in the ‘Installed’ tab, and the rest will go in the ‘Available’ tab.
Selecting any item from the list takes you to the details of that application. From there, hitting the menu key will give you the option to install, uninstall or update. You can also view (in the browser) to the application’s web site, issue tracker and source code, and you can also choose to view the application in the Android Market.
Additionally, if there are multiple versions available, you can select that version directly in the list and install it. You would use this, for example, for installing an older version or for installing a newer beta or nightly version. One version in the list will usually be shown with a * next to it – this is the ‘current’ version – i.e. the one the developer might publish to Google’s one-version-only market. Versions above this are probably beta releases, and versions below are older versions.