Free Online Readability Calculator - Flesch Kincaid
The code that calculates the scores displayed here has been released as an Open Source project: Text Statistics.
Several readability algorithms use syllable counting to determine their scores. The current syllable counter used here will only work with English text, so scores for other languages may be inaccurate.
The Coleman-Liau and Automated Readability Indexes do not use syllable counting, so should produce a reasonably accurate score in most European languages.