I develop on Mac OS X and frequently need to test my local build using Internet Explorer on a flavor of MS Windows. VirtualBox has supported an easy method of doing so since version 2.2
Add a new Network Adapter to each of your VirtualBox Guests (I run Windows XP and Windows 7 as Guests) and choose "Host-only Adapter" for the "Attached to:" option. This will result in the creation of a new loopback interface on your Host and a private network interface on each Guest.
On Windows use the command line "ipconfig" to determine the new interface settings. On Mac run "ifconfig" to do the same.
With my setup the host (my mac) was assigned the vboxnet0 interface with private IP address 192.168.56.1 so I punched a hole in my host's firewall to allow each Guest to access ports 80,443,8080,8443 on that subnet. Now I can simply open a browser on any guest and access the webserver running my local build on my host like - https://192.168.56.1:8443/