Kutub

This film is nothing less than a masterpiece.
It is a highly structured and stylized film about a serial killer.
It created the serial kill genre, which includes such entries as Psycho and Silence of the Lambs.
Alfred Hitchcock (the director of Psycho) was a disciple of Lang as were Jacques Tourneur (The Leopard Man (1943)) and Michael Powell (Peeping Tom (1960)).
M was not only the originator of the genre, but arguably remains it preeminent entry.

Highly recommended for those in the mood for a Hitchcockian-style thriller with a great performance by Peter Lorre and great story-telling technique by Fritz Lang.

M is in the public domain, and can be downloaded here for free:
http://www.archive.org/details/M_