Chrome: Google already uses its Chrome browser to warn users about potentially malicious software. Soon, the browser will begin blocking downloads that do annoying things, such as changing browser settings.
Presumably the new filters won't apply to the Chrome Store itself, nor will the blacklist include known safe software such as Firefox. However, the existing Safe Browsing feature -- which currently displays a red circle like the one above when it detects an unsafe download -- will be expanded to include "programs disguised as a helpful download that actually make unexpected changes to your computer."
That's not the download you're looking for... [Google Chrome Blog]