Using DOMCrypt, you have access to fast native encryption via a new window property: "window.mozCipher" - in all web pages.
'Native' refers to the fact that this addon is wrapping the native C-based NSS crypto routines for hashing, key generation encryption and decryption.
Update (2012): A W3C Working Group has been formed to specify DOMCrypt: http://www.w3.org/2011/11/webcryptography-charter.html
This API will probably become a standard via the W3C Web Cryptography Working Group. The API itself will no doubt change somewhat, moving to an event-based API with a more sophisticated DHKE symmetric API.
The homepage for DOMCrypt is http://mozilla.ddahl.com/domcrypt/demos/demo.html The Github repo is: https://github.com/daviddahl/domcrypt
DRAFT API SPEC: https://wiki.mozilla.org/Privacy/Features/DOMCryptAPISpec/Latest