A large, dark deskpad. It marks the area of my desk where only mouse and keyboard are allowed and other detritus is prohibited. Trying to use a mouse while subconsciously shoving junk aside can be done. But it's like coding while you have to pee: you won't be fully in the flow until you fix it (no pun intended).

The dark color was unexpectedly useful because it provides a clean and dark but well-lit area for scanning documents with my phone, including checks for deposit on a banking app.