Javascript Detection
It has always perplexed me how to detect when a browser doesn't have javascript enabled; not just installed, but also enabled.
Unexpectedly, while looking into favicons, I discovered this while looking through the HTML source on a particular website...
<noscript>
<meta http-equiv="Refresh" content="1; URL=/Errors/JavascriptNotEnabled.aspx">
</noscript>Wow! Surprisingly simple and graceful.
I've pasted this into the head of my master page and so far so good.