Collectible items that come in pairs - place them both and you can port back and forth between areas of your house! They can be mage-esque portals (the pink oval), floor pools (like Psijic or Nocturnal portals), fiery daedric spheres/gates, etc.
Let's say your guild hall is Bastion Sanguinaris and all of the crafting tables are way out on the terrace. You can put one end of the portal near the entrance and one out by the crafting tables so your guild members don't have to run all the way through multiple doors and huge rooms to get there!