Put storage around the periphery of the room. The lowest ceiling height is around the sides. If you block this area, no one is aware of just how low it actually goes.
Embrace the space. Slanted ceilings have their charm. Here the low angle creates a nook that feels cozy and all to yourself — just try doing the same thing in a vaulted room.
See what I mean? The clothing racks fill the area where the slant gets really low, so you can pick out your clothes with a buffer between you and the awkward end of the angle.
Turn it into a spare bedroom. You may not immediately think a bedroom would work for these tight spaces, but as long as you're not prone to jumping on the bed, it could very well fit just fine. Just make sure to test sitting up in bed — if you suddenly bolt upright you don't want to risk getting a concussion!
Play around with the arrangement. You might be surprised; sometimes it may look too low but in fact will fit just right.
Fashion it into a pass-through. You rarely hang out in hallways, so as long as there's a middle corridor where you can walk without bumping your head, it'll work just fine. This way it becomes an interesting architectural element rather than a detriment.
Pass it off to the kids. Turn an attic or other angled room into a play area that fits them just right. To a grownup it's an awkward space, but to a child it's their own kid-size hideaway or clubhouse.