Windows only: Free application Anti-Malware Tookit automatically downloads popular anti-malware applications to your computer. In essence the application is a specialised downloader that retrieves the latest and greatest releases of popular anti-malware apps like CCleaner, Spybot S&D, and Hijack This. You can selectively choose which apps you want downloaded and which you don't, so you can stick to the tried-and-true options and avoid the ones you're not sure of—perfect for when you've left your PC rescue kit at home and are working tech support for the family.
In addition to the anti-malware apps, the app also automatically grabs "recommended" apps like Firefox, Thunderbird, and avast antivirus (ostensibly to help the user keep a clean PC next time around), as well as utilities like Process Explorer and Unlocker. It'd be nice to see more options from our best antivirus list and a few more great Windows maintenance tools, but as is it's a good start. Anti-Malware Toolkit is freeware, requires .NET 2.0. It's not one you'll use every day, but it's a lot better than hunting down apps one-by-one on a malware-ridden PC.