This site, Responsive Logos, is something I've had sat in my bookmarks for a while. I normally browse sites on an iPad and this one really needs a browser you can resize to get the full effect. Most brands (especially the big ones) will have variations on their logos for use in various contexts; logos for giant billboards will be different from those used on tiny black and white guarantee cards, say. Online, the story can be a bit different, with any adjustment to the browser size or context just left to resizing the one logo graphic. Some are done differently, and I've made a few sites that switch to a simpler logo, but it's not that widespread from what I can see.
What Joe Harrison has done is take some big-name brands and simply and beautifully show how each can respond to a smaller browser size (or viewport, in the parlance). They all look like the real thing so I'm not sure whether he's created new images or got the smaller and simpler versions direct from the brand guidelines for each. It's a nice little site and a great demo if you ever have to explain it, to a client, say.
More fun are these Responsive Icons (or Rspnsv Icns) he's done, that you might remember from earlier in 2014.