This may be old news to some of you but as I've just encountered it I thought I would share it here.  Chrome offers a free (as much as anything is these days) add on of a basic form of Posterminder to remind you take breaks from the screen, have a walk etc.  It may not be as rich in functionality as some busiesses may require but for those of you with limited requirements and no budget it may be useful - just Google 'Chrome Postureminder' and it will come up with Chrome Store.

I don't get paid by them (I wish) I just thought it might be useful for some of you.