G'mar Hatimah Tovah" or "May You Be Sealed for a Good Year (in the Book of Life)."

This reflects the Jewish view of Yom Kippur as the day when God seals our fates (determined by our actions) for the upcoming year in the Books of Life or Death. The entire ten Days of Awe from Rosh Hashanah through Yom Kippur are viewed as the beginning of the New Year, so you may also still wish your Jewish friends a "Happy New Year" or "L'Shana Tovah" on Yom Kippur.