Anyone know what cultivar this might be?

No, it's Sedum kamtschaticum:
http://www.google.ca/search?q=...

Well that's funny, you're right it is sedum.

Was given to me by someone who thought it was euphorbia. She said to contain it in a pot sunken in garden so it didn't take over. That is what I have done for the past three years and it is thriving. Since it's not euphorbia hould I let it out of the pot in fall or spring?

http://garden.org/plants/searc...

SCButtercup said:Since it's not euphorbia hould I let it out of the pot in fall or spring?

Yes, it's a vigorous and tough plant but not invasive in the usual sense. You could plant it out at any time.... or at least I would do that here - do what's best in your climate.