VMware Server is free, but it asks for a serial number when installing. To get a serial number follow these steps:

1. Go to http://www.vmware.com/download/server/
2. Click "Download"
3. Read and accept EULA.
4. Download installation package if you already didn't. Notice that there is a link at the beginning of the page saying "Register for your free serial number(s) to start using VMware Server."
5. Click "Register" to go to http://register.vmware.com/content/registration.html and register to get a serial number.