A server software that attempts to block web scrapers that work around restrictions (by ignoring the robots.txt standard, masking themselves as end users, etc.) by making them solve a proof-of-work challenge before they are allowed to access the respective content. Solving this challenge is trivial for modern browsers, but difficult at scale or for scraping software.
Its current mascot, introduced in version 1.15.2, was drawn by CELPHASE.