#4: iPIG from iOpus
The VPN server and client from iOpus are free and open source, available for Windows. You won't receive any annoying offers or advertisements during the install of the client. However, you'll have to create an account to use the service.
Plus the free service is limited to a measly 10 MB of free traffic; then you must purchase more.
Once you exceed the traffic limit you could install and use your own free iPig VPN server.
After the installation, an icon in the system tray will appear. Double-click the icon to bring up the client application.
If you don't already have an account, you can click the shortcut button to create an account. After that, simply input your username and password, click iPig On, and you're secured.