On which sites is the citation helper enabled?
As far as I know, it was first introduced on MathOverflow: New feature - citation helper.
And it was later added to Mathematics Stack Exchange, too: Let's get MathOverflow's "Citation Helper" here, too!
Are there some other sites that use citation helper?
Just to clarify, citation helper is not the "cite" link which is below the post.
It is a button in the editor which opens a window that helps for searching and including citations to published articles.
You can find more information if you look at the links mentioned above. I have included a screenshot taken from the announcement on MathOverflow Meta:
