This plugin automatically adds upsell products to any product view. It also adds a possibility to set how many upsell products should be shown in the system configuration, which is not given in magento, if you're using random products or not. The upsell products are chosen based on the category the shown product is in. It searches only for products with a price higher than the shown products price and if not enough products are found it takes half the price and searches again and so on. There is also a configuration to exclude categories as a search criteria.