VirtualBox 1.6.4 was just released, fresh Downloads available now for many distros and OS's.

A few useful commands / tips for managing the Host side of VirtualBox.

Simply run the command

$ VBoxManage modifyvdi  some_image.vdi compact

However, this might not accomplish much, if the guest OS has "garbage" written all over the unused sectors. You can "fix" this by zeroing out all free disk space on all partitions, for example do something like inside the guest (assuming it's a linux OS):

$ sudo cat /dev/zero> /fill.it
$ sync
$ sudo rm -f /fill.it
$ sync
$ sudo halt

On a VDI I had which used 6.4GB on the host, and only 3GB inside the guest, doing the above shrunk the VDI down to 4.4GB. Not great, but a lot better

Here's a small section of some tips to get VirtualBox to work well with certain guest OS's.

In order to build the guest additions, you must install some required packages. I believe something like this would do it (run it as root):

$ apt-get install build-essential module-assistant
$ m-a prepare