A physical thing
Essential to your workspace are some good inspirational books. Nothing beats a good ol' stroll through a graphic design book. Soaking in all the awesome works of designers around the world can really help to start a project or get you out of a creative black hole.
All Men are Brothers is a really good one in this regard. A collection of 108 leading designers show their works in this 653 pages book. The book is always close to me for a quick dive into graphic design inspiration, it served already for many years and will continue to do that in the future.