We haven't covered every task that Osmosis can perform. Others, including tasks to generate reports about data, and to deal with legacy file and database formats, are also supported. You generally won't need to use any of these tasks while using the data, but they can be useful when trying to find obscure errors in the data itself. For a full listing of the tasks Osmosis supports, visit http://wiki.openstreetmap.org/wiki/Osmosis/Detailed_Usage.
Even if Osmosis can't yet do what you need it to do, you can extend it if you know enough Java. There's more information on writing your own plugins at http://wiki.openstreetmap.org/wiki/Osmosis/Writing_Plugins.