First, you should keep in mind that most of the holidays and short-term stays properties come with furniture while long-term rentals are usually unfurnished.
For holidays and short-term stays, you may want to check popular platforms such as Airbnb and Booking. If you are looking for a long-term rental, then you would better regularly monitor websites such as OLX and Imo Virtual.
Often, it’s also helpful to check the traditional real estate agencies such as Remax and Century 21, which, although they focus more on selling properties, might have places for rent.
Last but not least, word of mouth is still of value in Portugal, so don’t hesitate to ask your friends and people around you if they know of any vacant apartment around.