I have assumed the free game thread is more for permanantly free games. GOG.com has free games but usually only for a couple of weeks.
Constructor is a fun game. You play as a landlord/development manager and you have to build houses, keep tenants happy as well as the council while fending off rival factions. The game has a very british style of humour with punks, students and the like making up your tenants along with plumbers, football hooligans and psychos with chainsaws to send after your rivals. There's elements of "keeping up with the joneses" where if you give one tenant a tree, their neighbours will want one as well and threaten to move out if they get unhappy enough. Different tenants like certain objects as well. One I remember is nerd tenants liking white picket fences.
With all this said, however, the big point against the game is there is no tutorial. You play a skirimish game and pretty much have to learn on the fly how to allocate lots, construct factories for resources, collecting rent, keeping tenants happy and so on. It's important to keep in mind that you are primarily pitted against the council who will give you tasks to complete by set dates. Failing to do so usually ends in a game over or, I believe, a black mark that 3 of which will also lose you the game.
I would assume this going free for a bit would be because of the remake currently under work, apparently due to release on 28th of February.