DO not install Flash from anyplace but here (bookmark this page in your browser)
Download free Adobe Flash Player software for your Windows, Mac OS, and Unix-based devices to enjoy stunning audio/video playback, and exciting gameplay. The best known example of a fake Flash update is the Flashback Trojan horse, which first masqueraded as a Flash Player installer package targeting Mac users. Over 600,000 Mac users installed the fake update to Adobe Flash thinking it was valid, thereby infecting their machines with malware.
There are a lot of 'tricks' out there that try to get you to install Flash (or other plugins) and it's a trojan, or bad software that looks like it's ok to install.
This online check will assist to make sure your browser plug-ins are up to date, unfortuntly it's on Mozilla's site, but it works for all browsers
How To Update Flash Player
It's ok to install Silverlight if you need it, it's used for Netflix (bookmarkt this page in your browser)
Update Flash Player
Outside of that, Perian, Flip4Mac and VLC are pretty safe if you need to install them.
Please backup your personal data
Free Flash Player For Mac