A colleague of mine just sent me this link to freenigma:
freenigma uses one of the most famous and most widely used cryptographic software packages in the world: the GNU Privacy Guard (GnuPG)
It runs as a Firefox extension with an IE version in the pipeline. It's still early days yet - there's no encryption of attachments and digital signatures are disabled at present - but this looks promising. If you're interested, I'd recommend reading their FAQ. They've thought things out pretty well and are probably onto a winner here.
I've not been able to work out what type of licence the product is being released under. They're planning to offer this for free for individual use, which suggests a closed source :-(