This is a great idea for a plugin from Greg Reinacker. You set a minimum threshold for how many pageviews per minute would be “big” for you, and then if your site exceeds that threshold you get an email. It’s a clever way to wean yourself from that awful habit of checking your Mint stats every 15 minutes to see if you’re getting a spike of unusually high traffic or not.
And, if you don’t want to start checking your email every 15 minutes instead of your Mint stats, set the “send alerts to” email address to your cell phone provider’s text message address. This way you’ll get an SMS if you’re site starts getting lots of traffic, and so in the downtime you can stop thinking about it.