Unlocker

How many times have you come across messages like "Cannot delete file. it is being used by another person or program. Close any program that might be using this file and try again". This is a safety precaution of OS, more specifically explorer to 'lock' a file while a critical section is is accessing this file.

This software which can be downloaded from here is very useful in negating such things.
For example consider a file move operation on Windows. Following are the events which actually take place:
1. Source and destination are taken by the OS
2. A backup of the source file is created.
3. The move process starts and the destination starts getting created.
4. If the the process is aborted, the destination file is deleted and the backup file is placed in the source place.
5. If the process completes, the backup for source file is deleted and the destination file is created.

During the time when the process isexecuting, both the source and dstination file are 'locked' by explorer. This is to prevent any anomaly or integrity defect. Imagine yourself changing a file which is being moved. What should be the destination, the one which existed initially or the one which you created after your modification process?

The unlocking option is created as a context menu in explorer , visible on right click of the file/folder. This probably is acting as a hook into the explorer.exe process. It can also kill process, close open handles to a file and unload dlls.
All in all this is a very useful utility if you are doing very file intensive operations on the machine and frequently run into the problems where you run into problems mentioned above.
The usage is strictly discouraged just unless you encounter the problems mentioned as it changes the basic, default functionality of OS. In case you are dealing with critical files, which don't have back up, please dont use it. Since it is unlocking the files, there is (very small) probability that the file may become corrupted and you may not be able to recover.
All in all this utility targets one of the most common problems of Windows and solves it very nicely.
Cheers to unlocker.