Flash without installation

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

You should give "Click to Flash" a try. I believe that it does pretty much everything you are asking for.

This plugin disables flash in Safari by default, but to load some flash content all you have to do is click on the "Flash" button in a video and the video will load and play as usual. You can even whitelist sites in "Click to Flash" so you won't have to click on flash videos when browsing YouTube.

I have not been installing Flash at all on any of my machines. I use Chrome for YouTube, since it has a sandboxed version of Flash.

EDIT: Almost as soon as I hit Post on my answer I noticed that "Click to Flash has been superseded by "Click to Plugin" which appears to do much more. I haven't tried this, but it looks like the only choice for Safari 5.1 and later.