Cats Rule Freshen Up Filter Disc Pure Clean, 3 Pack

Have used them for at least 4 years. Works perfectly with two cats. Stay fresh.

These discs work in conjunction with the Cats Rule line of litterboxes, although I suppose you could rig these in place on some other brand of litterbox. They do help decrease the odor but they're not going to completely disguise it, either; if you're looking for something to more effectively deodorize a litterbox, you might want to add baking soda to the litter. Mostly I like these because they help me not to gag so much on the litterbox when I first open it up.