Field Collection provides a way to reuse sets of fields by defining an entity whose purpose is to have fields attached to it. Field Collection is very widely used in Drupal 7 builds. An alpha version of 8.x-1.0 exists and is working on Drupal 8.1.
Where's the code?
Alpha is available. ETA for a full release is unknown.
The Drupal 7 version of Field Collection depended on Entity API. All of Entity API's functionality is in Drupal 8 core, so the straight Drupal 8.x-1.x port has no additional dependencies (assuming its architecture isn't changing much).
The 8.x-2.x branch is rearchitected to depend on core's Entity Reference system and the Inline Entity Form module
Who's doing the port?
fago, larowlan, jmuzz and tim.plunkett are the main committers to 8.x-1.x. You can get some idea of when and how much each were involved up to October 2015 in the commit list on.
Currently tedbow is leading the work on 8.x-2.x, but is looking for help.
What help do they need?
Anyone familiar with Inline Entity Form, Field Widgets, and Field Formatters.