I have a dog that lets itself in through an external door. Is there a door closer that I could easily activate in winter (when I would like the door to close after the dog comes in) but which could be set to leave the door free in summer?

There are two ways of tackling this, depending on your budget.  The “deluxe” solution would be to use an electro-magnetic door closer (the Geze TS4000 EFS) with a PSU, the current to which would be switched ON in summer, so the door can free swing.  In winter you switch the power OFF and the door closer will operate to close the door after use.  The “economy” solution would be to go for a standard domestic overhead door closer, like the Eurospec F2003 (or have a look at our special offers page) and then when summer comes around simply remove the arm from the closer, replacing it in the winter when you want the unit to work.