This online tool breaks Vigenère ciphers without knowing the key. Besides the classical variant Beaufort ciphers and Autokey ciphers are supported as well.
As an example you can crack the following cipher text with this tool:
Altd hlbe tg lrncmwxpo kpxs evl ztrsuicp qptspf.
Ivplyprr th pw clhoic pozc. :-)
If you would like to know how this Vigenere breaker works have a look at the bits & bytes corner (German only).
If you want to break a monoalphabetic substitution cipher instead try the Substitution Solver.