I’ve used one called Uniplaces in Lisbon. They operate in large parts of Europe. Despite the name they are open to non-students as well (they call it “young professionals”). Only allow rentals of 28 days or longer. They do charge a fixed service fee per rental meaning it’s the same if you rent for one or six months, making longer rentals better financially.
Protip: Use the coupon “LANDINGJOBS” for 50% off their service fee.