OpenVPN is a newer and an outstanding VPN solution. It implements Layer 2 or Layer 3 connections, uses the industry standard SSL/TLS for encryption, and combines almost all features of the mentioned VPN solutions. Its main disadvantage is the fact that there are still few hardware manufacturers integrating it in their solutions. 

OpenVPN Version 1
OpenVPN entered the scene of VPN solutions only on May 13, 2001 with an initial release that could barely tunnel IP packets over UDP and only encrypt with Blowfish cipher and SHA HMAC signatures (rather secure encryption and signing methods). This version was already numbered 0.90—which seems ambitious, since only one version (0.91) followed in 2001, offering extended encryption support. For SSL/TLS support, users would have to wait almost one year after the first release. Version 1.0 was released in March 2002 and provided SSL/TLS-based authentication and key exchange. This version was also the first to contain documentation in form of a manpage.
Then, OpenVPN development picked up speed. Only five days later, version 1.0.2 was released, which was the first version with adaptations for Redhat Package Manager (RPM)-based systems. From this version on, releases were published almost regularly every four to eight weeks. 
The following table gives an overview of the releases and lists the dates and versions when certain selected features were added to the 1.x version of OpenVPN. More details can be found in the Changelog sections of the OpenVPN website at http://openvpn.net/changelog.html and release notes at http://openvpn.net/relnotes.html.

OpenVPN Version 2
Parallel to the improvement and development of OpenVPN version 1, the test bed for OpenVPN version 2 was made in November 2003, and in February 2004, version 2.0-test3 initially prepared the goal of a multi-client server for OpenVPN. This multi-client server is one of the most outstanding features of OpenVPN today; several clients can connect to the VPN server on the same port. On February 22, 2004, the two development branches 1.6-beta7 and 2.0-test3 were merged and further development was continued in version 2’s branch.
There were fewer than 29 versions labeled as "test" versions, 20 beta versions, and 21 release candidates, until on April 17, 2005, OpenVPN version 2.0 could be released. This was only possible because of the great number of developers contributing to the project, fixing bugs, and improving performance and stability permanently.
The following list will give a brief overview of the new features added to OpenVPN version 2:
• Multi-client support: OpenVPN offers a special connection mode, where TLS- authenticated clients (that are not blacklisted on the CRL) are provided in DHCP- style with IPs and networking (tunnel) data. This way, several tunnels (up to 128) can communicate over the same TCP or UDP port. Obviously, a mode control switch for activating server mode became necessary. 
• Push/pull options: The Network setup of clients can be controlled by the server. After successful setup of a tunnel, the server can tell the client (both Windows and Linux) to use a different network setup instantaneously.
• A management interface (Telnet) is added.
• The Windows driver and software have been improved widely.