Compute sales rank for products, either storewide, type, collection etc. Display as X/Y or a percent rank. Optionally create tiers for percent rank and instead show labels like "Super Hot", " Hot", "Popular". Compute over a period of time and/or all time. Exclude products you don't want ranked. Optionally set a cutoff to not show ranks of lower ranked products. Show this on collection page thumbnails or on the product details page.