This is an online version of "PyGenere", a small Python program that automatically finds the codeword for ciphertexts encrypted with the Vigenère cipher. You need to give the program a piece of ciphertext that's fairly long compared to the length of the key used to encrypt it.
The PyGenere source code is freely available under a 2-clause BSD-style license. If you have any comments or suggestions please contact the author (email address in source code).
The online version is limited to codewords between 1-20 characters long. If you need to get around this, download Python and run the program yourself; it should be quicker that way in any case =)
If you want a tool to encode texts to test here, Sharky's Vigenere Cipher is a good Vigenère encoder/decoder.