This page will check the potential strength of your password. It isn't a strictly accurate tool as a string of 'XXXXXXXXX' is not considered secure. As you type in a password, it will determine how many of each class of character there is, update the checks, determine the number of combinations that would fulfill the character combinations and, at various attempt rates, calculate the amount of time that it would take to bruteforce the full range of characters available.
The script is embedded within the page so that it is completely transparent and passwords never leave the browser, which can be verified by looking at the source.
The estimates are estimates and shouldn't to be taken literally due to the size of some of numbers involved with the larger numbers and the limitations of JavaScript.