NB The following is a community-generated list of websites that republish Stack Exchange content without attributing it properly. It is no longer being maintained, because the procedure for reporting such sites has changed; see the duplicate for more information.
There are a number of license-violating clones of Stack Exchange sites popping up that use Stack Exchange's CC-wiki data without following our Creative Commons attribution terms. Those terms are linked at the bottom of every Stack Exchange webpage, and are also included as a
.txt file in every data dump we produce.
Google now has a form for reporting content copiers that appear above the originals in search results.
The option to block a site appears when you click a search result and then navigate back to the search results page. Click the "Block" link next to that result to block all pages within the site's entire domain.
This post was created as an attempt to organize information originally posted in this answer. You may also be looking for the post for reporting copycat sites that outrank Stack Exchange in search results.