This is a simple web-app I made ages ago that encrypts and decrypts simple text with a password.
It uses the mcrypt class in PHP, and the data is passed via AJAX.
EDIT (10th June 2019): I’ve added a security certificate (thanks to Let’s Encrypt SSL), due to popular demand. Make sure you are visiting the correct URL.
App can be found here: https://dev.ramin-hossaini.com/encrypt/