Adobe has updated Flash Player (which you interact with primarily in your Web browser) to version 10.1.85.3 to address a security vulnerability [1] that's actively being exploited in the wild against the Windows version of Flash Player (as of this writing, Macs aren't being targeted). To determine the version of Flash you're currently running, visit the About Flash Player page [2]. If you need a new version, visit the Adobe Flash Player Download Center [3]. If you're using Google Chrome [4], version 6.0.472.62 includes the new version of Flash Player and has probably updated itself automatically. (Free, 7.59 MB)
[1]: http://www.adobe.com/support/security/bulletins/apsb10-22.html
[2]: http://www.adobe.com/software/flash/about/
[3]: http://get.adobe.com/flashplayer/
[4]: http://www.google.com/chrome/