Anubis is a simple anti-scraper defense that weighs a web client's soul by giving it a tiny proof-of-work workload (some calculation that doesn't have an efficient solution, like cryptography) before letting it pass through to the actual website. The workload is insignificant for human users, but very taxing for high-volume scrapers. The calculations are done on the client's side using Javascript code.

(edit) For clarification: this works because the computation workload takes a relatively long time, not because it bogs down the CPU. Halting each request at the gate for only a few seconds adds up very quickly.

Recently, the FSF published an article that likened Anubis to malware because it's basically arbitrary code that the user has no choice but to execute:

[...] The problem is that Anubis makes the website send out a free JavaScript program that acts like malware. A website using Anubis will respond to a request for a webpage with a free JavaScript program and not the page that was requested. If you run the JavaScript program sent through Anubis, it will do some useless computations on random numbers and keep one CPU entirely busy. It could take less than a second or over a minute. When it is done, it sends the computation results back to the website. The website will verify that the useless computation was done by looking at the results and only then give access to the originally requested page.

Here's the article, and here's aussie linux man talking about it.