http://blog.malwarebytes.org/fraud-scam/2013/11/potentially-unwanted-miners-toolbar-peddlers-use-your-system-to-make-btc/You're already used to "desktop toolbars" and similar downloaded crap that use a lot of CPU and memory and slow your machine to a crawl. Now some folks are taking it to the next level.
You download their app, it does some minor trivial thing badly for you, does the usual spyware thing where it reports your web surfing and shopping habits to the home base, and also downloads mining software in the background and then spends 99% of your CPU/GPU mining bitcoin. Not that you actually get anything if it hits a block; that all goes back to the guys distributing the app.
So... This is a trickle of hash power in a big bucket considering the ASIC farms that are out there, but the fact that they're trying to be sneaky about it and weaselwording their EULA so it could be construed to give them permission to do so is new.
