VirtualBox has been great for me, but I thought I'd give KVM/qemu a try. I'd tried running Bochs around 2003 and it was exciting to see that it actually worked, but the poor performance made it more of a novelty than a useful tool. It's nice to see how far Qemu it's come in this time. It's not only usable but quite handy, with simple CLI management and excellent performance.
VirtualBox has a few options for network access. By default VirtualBox NATs the guest OS out to the network via the host OS, so the guest can reach the internet but outside hosts can't initiate sessions to the guest. One option for running a server from the guest OS is probably to bridge a virtual interface with a physical one. Newer versions of VirtualBox also have the option of configuring port forwarding, which is what I'm going to write about here.
VirtualBox is a very accessible open source virtualazition software platform. When combined with the optional free-as-in-beer guest additions, it makes for some really slick guest/host desktop integration. In my experience it is very stable and the guest machines seem to operate at native or near-native speed. While the underlying platform is excellent, the management tools aren't as advanced as what, say, VMware has to offer.