SSL VPNs are the future of VPN technology. While they are still brand new, "bleeding edge" sort of technology, they will eventually be how we run our VPN connections for most organizations. The concept is simple: HTTPS (SSL-based) web pages have used adequate encryption for years…why not harness the technology to create a "client-less VPN system," tunneling applications through the SSL connection.
For a user to connect to a SSL VPN, no client installation is necessary. Rather, they simply access a web page, authenticate, and minimize the web browser window. They're now on the corporate VPN.
There's more to it than this (such as JAVA client downloads may be necessary for full port forwarding capabilities, etc…). Cisco just published an excellent explanation / configuration document for the WebVPN/SSL VPN technology. Get it here.