Flash plugin crashes frequently
There's a reason why Apple appears to onlookers to be so anti-Flash, and it's because Flash sucks on Macs (it may also suck on other OSs, but I don't use them enough to be knowledgeable). 

What to do about it depends on what browser you use (your question doesn't say, as Mozilla's Plugin Check isn't FF-only):


Safari - the answer is ClickToFlash. Honestly, C2F is something that should just come pre-installed on every Mac.
Firefox/Camino - As Ken mentioned in a comment, you want the FlashBlock extension.
Chrome - use the FlashBlock for Chrome extension.


As has already been said: you can get along fine on almost all sites without Flash. Here's some additional recommended reading, both of which are from John Gruber at Daring Fireball:


Going Flash-Free on Mac OS X, and How to Cheat When You Need It
Masquerading as Mobile Safari to Get Websites to Serve HTML5 Video to Safari on Mac OS X