Triffids are the original man-eating plant.
They first appeared in John Wyndham's 1951 novel, The Day of the Triffids, and later appeared in Simon Clark's 2001 motion picture, The Night of the Triffids. Basically, they are semi-intelligent, predatory plants, with venomous stingers. They also produce an oil which turns out to have myriad uses. They are vulnerable to sea water, the touch of which causes them to dissolve (much like sprinkling salt on a slug or snail).
It is never fully explained where the triffids originated - in Wyndham's novel they are the result of genetic experimentation, but in the later film adaptations they are thought to be extraterrestrial in origin, and in the 2009 BBC miniseries, they are said to be a naturally occurring species from Zaire.
The Day of the Triffids is considered part of the Golden Age of sci-fi.
Thanks to Oolong for ideas on improving this writeup.