To Download the iso-image of actual ESXi, you have to complete the free registration on vmware.com.
Registration is strict required otherwise you will be not able to download the installation media and the free license.
Some days ago I started to give up my VMWare Workstation Server (version: 6.5.1 build-126130) that is running on Debian Lenny.
The reason was it’s very difficult to upgrade (you see my installation is a little bit outdated:)) and maintain.
The first step is to migrate to VirtualBox running. And I wrote this little tutorial to migrate VMWare VMDK Image to VirtualBox.
File -> Virtual Media Manager