Colleague found a pretty brilliant tool: SSL Explorer, a small https-server that together with a Java applet in the browser implements a VPN. Specifically, when the applet starts (which must be confirmed, as the applet requires additional capabilities), tunnel connections are established over https, and various applications are then integrated over these connections. For example, you can establish a VNC connection to an internal server with a click on a link, browse the local Windows network via web forms, transfer files, or access Linux servers behind the firewall via SSH. And the whole thing works with a simple Java-capable web browser - I tested it with Safari, for example, and it works flawlessly. Completely without additional client software to be installed. Ideal for roaming users who don't always have their own device with them.
Oh, and the whole thing is also under the GPL.