Health. This is essentially a helmet that allows you to take a hit during your run without stopping it. It begins with the Rusty Bucket, but can be powered up all the way through the King's Crown.
Damage. This strengthens your axe over the course of your run, allowing you to take down tougher or shielded enemies with ease. You'll start with the Throwing Hatchet and work your way up.
Axe Speed. It's exactly as it sounds, allowing you to increase the velocity of your throws. You can also improve Axe Range as well, expanding how far you can throw them.
Magnet. It helps you draw coins close to your character. This is very useful when it comes to coin showers, as you'll really clean up when they rain down.