Hardly substitutes for MAC's Jet, but for those who enjoy a darker lip...
...another fun dark lip color is Illamasqua Violate Sheer Lipgloss - dark forest green shimmer in a sheer almost black base. Christine of Temptalia has swatches and other photos:
http://www.temptalia.com/illamasqua-violate-sheer-lipgloss-review-photos-swatches
It appears that sephora is not carrying this color... or maybe just not yet...
Plus quite a few quality lines have full-on black lipstick, including Illamasqua, not so many have a meltallic black / grey, though. Urban Decay Oil Slick is a bit sheer.