L0pht (pronounced "loft") was a famous hacker collective located in the Boston, Massachusetts area between 1992 and 2000. Some of the L0pht's members, including two of its original founders, were also members of the Cult of the Dead Cow.
In time, the L0pht members quit their day jobs to form a business venture, named L0pht Heavy Industries. This hacker thinktank released several security advisories and produced some widely-used software tools, like L0phtCrack (a password cracker for Windows NT).
In January 2000, L0pht Heavy Industries merged with the computer security startup @stake.