I can't choose just one so here are my favorite desk things...
In order of appearance:

Coffee, because no dev can dev without.

Mini whiteboards, (one on each side), makes for easy quick notes and helps me organize my thoughts.

Legos, specifically #4070 because of its intriguing geometrics. Tearing them apart and making different shapes helps me think through problems.

Code keyboard, pure excellence.

Logitech MX master mouse, same as above.