To go one better, there’s http://isthereanydeal.com/, which tracks prices across a bunch of vetted key retailers (i.e. companies that buy wholesale keys from publishers and sell them to users, but not grey-market or dodgy sites) so you can see where’s cheapest and get notified of discounts etc.

Why check GreenManGaming and Steam (and potentially a bunch of their competitors, too) when you could check one site and know who’s best?

I’ve accidentally made this read like an ad, but they’ve not paid me to say this, I just always check the site before buying games, and have either saved loads of money by doing so over the years, or have ended up buying a bunch of things I’d have ignored on the grounds they were too expensive otherwise. I don’t know in which direction, but it’s definitely changed the amount I’ve spent on games over the last ten years.