The S'well Blender Spring 2-Pack ($10) turns any bottle into a protein shaker. I was very sceptical that it would blend anything, but I was pleasantly surprised. The slender design of the spring allows it to fit into every reusable bottle I have — including the pictured Skye Traveller ($30) — so I don't have to worry about making sure I have the correct bottle for the spring.
Tip: If you're using a S'well bottle, make sure to leave a bit of space under the inner lip when you fill the bottle so everything gets evenly blended.