On some Macs, Flash isn't just a necessary plug-in that's occasionally crash-y. It can be a lap-heating, fan-speeding, battery-draining headache that sticks around longer than you'd like. Chronicler of all things Mac John Gruber provides a guide to going almost entirely Flash-free, with a few cheats here and there.
In short, Gruber suggests removing Flash components from the plug-in library, installing the (previously mentioned) YouTube5 Safari extension for YouTube and its embedded videos, and then, when Flash is the only way through, loading up an instance of Chrome and shutting it down right after, so Flash doesn't "stick around". Read his post for the full walkthrough and justification of the effort.