Main changes:

• Multiple tools are now grouped together so that they can be more easily accessed by their numeric shortcut keys
• Custom tool layouts can be created, and cycled between with Space
• Quick swap now works a little differently since the previous behaviour seemed a little confusing at times - it now acts as a second slot instead of the swapping between the previous and current tool
• Impact splatter as suggested by @OBAMAGAMER
• Added the knife
• Added the staple gun

Minor changes:

• Objects thrown fast enough will temporarily have more accurate physics so that they don't go through other objects.
• Added a button in the pause menu to open the screenshot folder
• The tool palette/UI now has a single solid background across the entire screen
• Spray paint shake sound doesn't play when the tool palette is open
• Basic button changed to "Classic" in main menu
• Changed debug tool modifier from space to ~ (tilde)
• For debugging ~ + PageUp toggles between basic and sandbox mode

Changes to Tools

Common tools are now grouped to allow more to be accessible by keyboard shortcuts, which I thought would be important as I add more tools in the future.

Although the look and behaviour of the tool UI isn't very different, it took longer than originally planned to implement.
The reason being that the original code had gotten a little messy, so I took the chance to rewrite the UI code from scratch, as well as rewrite and refactor some of the tool related code.

The Tool Layout Editor

Another feature I thought might be useful is allowing the user to create one or more custom tool layouts.

This way the user can choose which tools are grouped together, and in what order.
They can be individually enabled and disabled, and by default the Space key can be used to quickly switch between enabled layouts.

Impact Splatter

It's not perfect, but bloody body parts can now leave splatter on walls and other things they touch.

Getting the basics working was easy, but tracking objects that touch and slide along each other was unexpectedly difficult.
To prevent to objects resting on each other from constantly creating splatter, individual contact points need to be tracked every frame, and only when they move by a significant distance will that be considered to potentially create splatter.

I tried to limit it to a reasonable amount, but it still means extra processing and drawing every frame, so there are two new graphics options.

• Impact Quality - will adjust how frequently splatter can happen.
  Additionally Basic mode will only create splatter when two objects first start touching, so there won't be any need to process collisions every frame.
• Impact Objects - What objects can splattered on. The options are everything,  or just walls which will disable impact splatter on other body parts.

Added New Tools

The knife can be used to make small cuts along the body, and staples can join two body parts together.