Best Buy offers the EveryDrop Whirlpool Refrigerator Water Filter 4 for $49.99. Check out via Easy Replenish to cut that to $39.99. With free shipping, that's the lowest price we could find by $5. It claims to reduce 23 contaminants including pharmaceuticals, waterborne parasites, lead, mercury, and pesticides.

Note: Easy Replenish is akin to Amazon's Subscribe & Save service; be sure to cancel your subscription once your order ships to avoid being billed for further shipments.