Are Mezuzas Jewish?

Yes, mezuzas are Jewish. Affixing a mezuza (aka mezuzah) to the doorpost of your house is a prominent commandment to the ancient Israelites in the bible, prominently specified in verses recited by Jews in the Shema prayer twice daily (e.g., Deut. 6:9).

But the Times reports that plenty of non-Jewish people, especially in Brooklyn NY, inherit mezuzas when they move into their houses or apartments.

In a wonderful, warm article the Times notes about those Gentiles who keep their mezuzas:

...But many gentiles choose to keep their piece of Judaica in place.

“It’s good karma, if I can mix my religious metaphors,” said Brian Hallas, a resident of Kensington, Brooklyn, who teaches kindergarten at the Calhoun School in Manhattan. Although his mezuza was heavily camouflaged in what he described as a “lovely institutional beige” hallway tone, he spotted it immediately upon moving in, having once received a mezuza necklace from a college sweetheart... more...

Hat tip to Yochanan Hashlishi.