This German VPN service provider allows you 10 GB of transfer per month with its free service which is quite a lot! The paid service is pretty affordable as well.
Though it advertizes itself as an application for web anonymity, it is a VPN client specializing in that job. There is an application available for the iPhone as well!
The AN.ON (ANonymity ONline) project aims to be a mix between a proxy and a VPN service to preserve your anonymity online. This project will remain free forever.
For those looking for configuring your own VPN server on Windows XP, here is a quick and easy way to go about it: VPN setup.