signing data inside your browser?

Let data be textarea->value and browser be (firefox or chrome).

I want the user to be able to sign the data he entered in the textarea as I do not trust the website to store the data without modification.

So far I found a couple of GnuPG/PGP based solutions:

Didn't test any of them yet, so I am asking you, dear Lazyweb: are these any good? Are there any more such tools? What about X.509 client certificates? Can I abuse them for signing in the browser too? So far I found login stuff only. Pointers highly appreciated.