Always recycle them. Leftover ink can seep into the environment, and the plastic takes centuries to decompose. Many ink-cartridge companies run recycling programs. (Check the relevant company’s website for mail-in details.) Or use the search tool at earth911.com to find businesses near you that accept old cartridges. Office-supply stores, like Staples and Office Depot, offer rewards for returning all types of empties.
After collection, the cartridges are generally made into other products (such as new ink cartridges or clothing hangers), or they’re turned into energy to power homes and businesses. If you have an HP printer, check out HP’s Instant Ink replacement service. It detects when ink is running low and sends new cartridges along with a prepaid envelope for returning the used ones. Plans range from $3 to $10 a month.
Something else to consider: Epson’s line of EcoTank printers use a refillable ink reservoir instead of a cartridge, eliminating a lot of plastic waste.