I've used Sun VirtualBox (Now Oracle VirtualBox) for a couple of years as a handy way to run Linux on my Windows-based laptop.  It broke a while back and I only just got around to reinstalling it using the new 4.0.0 release - and found shared folders didn't work out of the box.  I could set one up - and I could see it - but it wouldn't allow me access - I got the following error:

You do not have the permissions necessary to view the contents of "sf_virtualbox_sharename"

As part of my C programming course recently I needed a Linux environment to work in, so decided to give Sun VirtualBox a try as a way to run Ubuntu 9.10 on my Laptop (running Windows 7).  It works well, but required a little tweaking to get going properly.

First, you need to install guest additions - to do this you need to choose it from the devices menu at the top of the VirtualBox window... this mounts them as a virtual CD ROM drive in your Linux install.