Intersect Contribs
Wikimedia Tool Labs — Pietrodn's tools.

This tool intersects the contributions of two or more users on a given WMF project, showing the pages edited by all of them.
It can help in discovering sockpuppets.

Update: On 2016-04-05 I updated the tool to support the intersection of the contributions of multiple users. Unfortunately, to handle this in a reasonable way, I had to change the format of the URL parameters. The old URL format ("user1" and "user2") is still supported, but eventually this will be removed.
This means that existing links to the tool referring to a specific couple of users will be broken.
Some parameters are missing.