Madrid has many art spaces owned by big businesses and run as non-profits. Fundación Mapfre (Sala Recoletos) is one of my favourites, mostly because they have excellently curated exhibitions that are always very well lit and hung. At a reasonable 3 euros, I can think of few better ways to spend an hour or so.
It is near Plaza de Colón, so very central, and housed in a really beautiful building. The ticket office is in the courtyard, but buying online is a good idea if you plan to go on a weekend, as it gets busy on Saturdays and Sundays.
There is a new exhibition running until the 25th August of the works of photographer Berenice Abbott, which I plan to see soon!
In the basement there is a permanent collection of works by Catalan genius Joan Miró, who could do no wrong in my opinion! There is also a very attractive shop to buy postcards or prints of your favourite pieces.