Discussion on:

Create VMs with downloaded virtual appliances in VirtualBox and VMware Player

Jack,
It is even easier than creating a vm and plugging in a disk as you mention above.
The standard way for distributing virtual appliances is to use .ovf or .ova files which adhere to the Open Virtualization standard.
There are some over here: http://www.oracle.com/technetwork/community/developer-vm/index.html

To install these, you simply double click on the .ova or .ovf file and VirtualBox sucks it in.

Cheers,
-FB