Ask HN: How do you collect training data for your Machine Learning / AI projects

I am looking for services ML researchers (or hobbyists) use to get training data for their models.

If you use one of the popular services like Amazon's Mechanical Turk, or Crowdflower, what kind of tasks do you submit to those services.

Are tasks typically a classification tasks, where user is presented with a set of classes and is required to select one (or more) classes that identify the task at hand.

I am collecting some data, as part of a project. Please be kind, and let me know if you any of what I asked doesn't make any sense.