What about lamium maculatum (spotted dead nettle). It grows really easily, fills in quickly, is a pretty pale pink flower with green/silver leaves and can be easily pulled out if it gets into places you don't want it. There are different varieties and flower colors, white and yellow. Doesn't mind the drier areas, at least mine is thriving under my cedar hedge. And its not very tall which I like. Unfortunately I can't find a nice pic of the pink flowered one but this link (from Seattle area) has a gorgeous one of the white flowered version

Here is a link that might be useful: spotted dead nettle 'white nancy'

I agree with the lamium. I have a pretty lavender one. When it starts filling in past where I really want it, I just pull some of it up, and transplant it where I want more. Some of those I transplanted reverted back to white, while others stayed lavender. They are all pretty, with their light green/white/silver foilage. And it comes back every spring. In fact, it stays pretty even under the snow/harsh winters here in Nebraska.
Leslie