VirtualBox is partially libre, gratis virtualization software.
VMware Support
While VirtualBox should be able to open .vmdk images, you may need or want to convert the formats on the command line. Also, you can crack open VMware images with dd and loop-mount the partitions individually. See the VMware article for instructions.
Converting a Flat VMDK to a VDI
To convert a flat VMware image to a virtual desktop image, you just need to run the VBoxManage convertfromraw command.
$ VBoxManage convertfromraw CentOS/Red\ Hat\ Enterprise\ Linux\ 4-flat.vmdk ~/RHEL4.vdi VirtualBox Command Line Management Interface Version 3.1.0 (C) 2005-2009 Sun Microsystems, Inc. All rights reserved. Converting from raw image file="CentOS/Red Hat Enterprise Linux 4-flat.vmdk" to file="/home/user/RHEL4.vdi"... Creating dynamic image with size 3221225472 bytes (3072MB)...