Hi shahid khan,
Linux does not need defragmentation of the hard drive. Different file system than NTFS. There might be a tool available that maps out the free space in detail by following the inodes in the file system. Try downloading efs2progs from the
sourceforge website and after installing explore the man pages for its separate tools. Join the mailing list and ask the question there for users-ext3 (which is very active), and the file system experts will either suggest a tool, or inform you how to find out the Unassigned (free) space, and distinguish why Linux does not need defragmentaion.
-- Tom