Vulgarization:

The Elegant Universe by Brian Greene (book)

Beyond String Theory by Jan Troost

String Theory on Wikipedia by various authors

The Elegant Universe by Brian Greene and PBS

Historical Introduction to String Theory by John Schwartz

Superstrings by John Pierre

String Theory introduced at Paris Ecole Normale Superieure

String Theory introduced at Cambridge DAMTP

String Theory Website by Patricia Schwartz

Introductory books:

A first course in String Theory by B. Zwiebach

Reference works (advanced):

Superstring Theory (2 volumes) by M. Green, J. Schwarz and E. Witten

String Theory (2 volumes) by J. Polchinski

String Theory in a Nutshell by E. Kiritsis

Books (advanced):

String Theory and M-Theory by K. Becker, M. Becker and J. Schwarz

D-branes by C. Johnson

An Introduction to String Theory and D-branes by R. Szabo

Gravity and Strings by T. Ortin

Lectures on String Theory by D. Lust and S. Theisen

Recommended on-line resources (advanced):

Introduction to Superstring Theory by E. Kiritsis

On-line resources (advanced):

What is string theory ? by Joe Polchinski

String theory on Calabi-Yau compactifications by Brian Greene

High energy physics database (SPIRES) by SLAC

The Arxiv (the database of recent papers) by P. Ginsparg at Los Alamos / Cornell

Other resources (advanced):

A pen, paper, and a computer with software, by various authors