Some companies, like Hewlett Packard, recycle their printer cartridges directly. Each printer cartridge comes with a UPS label for easy return. If you can’t find yours, go to their site.
Another option is to collect your cartridges and donate them to a local school running a drive. If you’re looking for a drive near you, or want to set up one, check here.
You can also drop off your cartridges at most Staples stores and get $2 in Staples rewards.
Note that it is now possible to get your printer cartridges refilled at places like Costco, though printer makers like (who make a lot of money selling those cartridges) claim the quality will go down markedly, and use of third-party cartridges often invalidates the warranty on your printer. Many people swear by this method, however, and it is significantly cheaper.