VMware Labs released a new Fling called Guest Reclaim. The download is available as zip and the total size is about 1MB.

Quote taken from the Fling page:

Guest Reclaim reclaims dead space from NTFS volumes hosted on a thin provisioned SCSI disk. The tool can also reclaim space from full disks and partitions, thereby wiping off the file systems on it. As the tool deals with active data, please take all precautionary measures understanding the SCSI UNMAP framework and backing up important data.

Features

• Reclaim space from Simple FAT/NTFS volumes
• Works on WindowsXP to Windows7
• Can reclaim space from flat partitions and flat disks
• Can work in virtual as well as physical machines

System requirements:

Supported Operating Systems:

• Desktop: XP onwards up to Windows 7
• Server: Until Windows 2008

Administrative Privileges:
The tool needs to be executed with Administrator privileges.

Whats a Thin provisioned (TP) SCSI disks?

In a thin provisioned LUN/Disk, physical storage space is allocated on demand. That is, the storage system allocates space as and when a client (example a file system/database) writes data to the storage medium. One primary goal of thin provisioning is to allow for storage overcommit. A thin provisioned disk can be a virtual disk, or a physical LUN/disk exposed from a storage array that supports TP. Virtual disks created as thin disks are exposed as TP disks, starting with virtual Hardware Version 9. For more information on this please refer http://en.wikipedia.org/wiki/Thin_provisioning.

What is Dead Space Reclamation?

Deleting files frees up space on the file system volume. This freed space sticks with the LUN/Disk, until it is released and reclaimed by the underlying storage layer. Free space reclamation allows the lower level storage layer (for example a storage array, or any hypervisor) to repurpose the freed space from one client for some other storage allocation request. For example:

• A storage array that supports thin provisioning can repurpose the reclaimed space to satisfy allocation requests for some other thin provisioned LUN within the same array.
• A hypervisor file system can repurpose the reclaimed space from one virtual disk for satisfying allocation needs of some other virtual disk within the same data store.

GuestReclaim allows transparent reclamation of dead space from NTFS volumes. For more information and detailed instructions, view the documentation PDF.