Marimo Moss Ball (Cladophora sp.)

Marimo isn't a moss at all, despite the name everyone uses for it — it's actually a ball-shaped colony of Cladophora algae that grows into that shape naturally, slowly rolling along lake bottoms in a handful of spots in the world, rotating just enough that light reaches every side evenly. In a tank, it's about as close to a completely undemanding plant as freshwater keeping offers: low light is fine, no substrate or fertilizer needed, and shrimp will actually graze on the biofilm that collects on its surface, which is a nice bit of free cleanup crew real estate. Give it an occasional squeeze and turn under water to clean out debris and keep its shape even, and it'll sit happily in a tank for years, if not decades.