What is this crawling next to my house?
It looks as though it has a keel along it's back (the area behind the mantle shield.)

If so (and I think it is), it would be a keelback slug, the coloration strongly suggesting a leopard slug (Limax maximus):



Coloration varies but the general pattern is a spotted spotted mantle sheild about a third of its length in size, with a striped tail.

The major doubt I have is the size. It looks smallish in the photo. Leopard slugs are very large. Perhaps it's a juvenile.