Could be Hosta 'June', but there are so many hosta. Try posting on Gardenweb's Hosta forum if my suggestion doesn't look quite right.

Here is a link that might be useful: H. 'June' images

I think carrieb is right. Here is my 'June' to compare. She gets a little sun in the afternoon:
{{gwi:249379}}

You found a winner in your garden. Hosta 'June' is one of the most striking plants.