This post is now old, but the crypto-js, may be now the most complete javascript encryption library.
CryptoJS is a collection of cryptographic algorithms implemented in JavaScript. It includes the following cyphers: AES-128, AES-192, AES-256, DES, Triple DES, Rabbit, RC4, RC4Drop and hashers: MD5, RIPEMD-160, SHA-1, SHA-256, SHA-512, SHA-3 with 224, 256, 384, or 512 bits.
You may want to look at their Quick-start Guide which is also the reference for the following node.js port.
node-cryptojs-aes is a node.js port of crypto-js