Another plant that I got in the Plant Swap with my sister last year which has done absolutely beautifully is yarrow. Here's what it looked liked when I added it to the side border in June of 2007. It's at the very bottom of the photo, tucked into the corner at the near end of the garden:
And here's how it looks this year:
The yarrow has a wonderful cottage garden feel: the delicate flower heads that look a lot like Queen Anne's Lace, the abundant lightweight foliage which sways in a breeze, and the sense that it nearly overflows the garden. This is another new perennial that I just couldn't be happier with.