Cryptico.js is an easy-to-use JavaScript library for encrypting text on the client-side.
It has support for RSA + AES methods and the text can be encrypted with any given bit length (228, 1024, etc.).

The content is encrypted with a public key and it can only be decrypted with that key (which makes sense if the recipient already has that information).
Cryptico.js doesn't require any JS frameworks to function and it is well-documented.

Requirements: No Requirements
Website: http://cryptico.wwwtyro.net/