Chrome includes a built-in PDF viewer and Adobe's Flash Player. That takes care of two major security threats in one swoop. Google keeps these always up to date with the latest patches.

Additionally, Chrome includes access to the Chrome Web Store. This is a collection of useful free and paid add-ons.

Google has also provided an extension to block tracking by advertisers. It is called Keep My Opt-Outs. Click here to learn more about it. 

Cost: Free

Windows: google.com/chrome 

OS X: google.com

System: Windows, OS X