Furman PST-8. It's not a plug in but provides better noise filtering than most, down to ~ 3kHz and exceptional surge protection.
Smaller plug-in AC Purifiers -- Shunyata, iFi, Puron...something else? What do you like?
I like the Shunyata Venom Defender. It does a good job of reducing noise in my outlets.
I'm wondering if others have tried similar, plug in products — such as the Puron Power Filter or the iFi AC Purifier or something else.