Lead Magnet
A lead magnet is a valuable piece of content or a tool offered free of charge in exchange for a visitor's contact details, usually an email address, in order to build a list of potential customers.
In practice
The most effective lead magnets solve one narrowly defined problem very well, rather than trying to be a broad, general resource. A lead magnet should also be closely related to the actual offering, so that the contacts you gain are genuinely relevant prospects.