This is a picture of diatomaceous earth, also known as diatomite or kieselgur, as viewed under bright field illumination on a light microscope. View the full image (7000px wide) on Wikipedia and explore it! The diatom particles are in water and the image is covers a region of approximately 1.13 by 0.69 mm.
You won't have heard of diatomaceous earth, but you will have used it! It also looks amazing under a microscope. Diatomaceous earth is a soft, siliceous, sedimentary rock made up of the cell walls/shells of single cell diatoms and readily crumbles to a fine powder. It is used for cleaning (scouring), filtration, heat-resistive insulation, killing headlice and as an inert absorbent substrate. Its most famous use was by Alfred Nobel who developed dynamite; a mixture of diatomaceous earth and nitroglycerin! Diatom cell walls are bivalve, i.e. made up of two halves, and are made up of biogenic silica; silica synthesised in the diatom cell by the polymerisation of silicic acid. The two main groups of diatoms are centric (radially symmetric) and pennate (bilaterally symmetric).
Make sure to explore the image properly, there are so many fossils to see!