I write a message to web webmaster@ and tell them which version of Netscape I;m using and which page krashed. Be brief and blunt, but be nice. The last site that klunked me was NEC. I haven't been back since.
If the issue isn't resolved after an exchange of messages, I make a point of telling them I have put a purchasing freeze on the offending vendor's products. Compatibility with Most browsers is not a lot to ask from a hardware vendor.
A buddy of mine still goes to <mega_canadian_consumer_electronics_retailer> whenever he buys new goodies somewhere else just to drop off a copy of the sales receipt. "See? There's another $600 I didn't spend here!"
They screwed him over real good back in '98.
Now if it's some pr0n site that's krashing you, complaining will get you on every sticky SPAM list out there.