I'm a big fan of slugs! This is either Arion ater or Arion rufus the Black and Brown slug respectively. Both species can be very variable in their colouration.
The matter of identification can be pretty moot with these fellows. This fact sheet explains a lot of the differences
Have you ever seen my favorite slug Limax maximus The leopard Slug ? Check them out, not only facinating but not even a garden pest.