Are there any tools to periodically check if a web site is available?

Either online or local applications (Windows XP preferably). I'm specifically looking to be notified by email if the web site in question goes down.

I've tried the Firefox add on, Mr Uptime, but it doesn't seem to have any options for polling frequency. There's also Pingdom who appear to produce this add-on and their online service of the same name.

Does anyone know of anything else which could fit the bill?

You already mentioned pingdom.com

The good thing with pingdom is that they are the only free service I know of that offers a 1 minute check resolution. Their free service is restricted to one site, if you want more checks you'll either have to pay or participate in GIGRIB. All other services I found are either paid or their they check only every 5 or 10 minutes.

EDIT: site24x7.com also offers to check every minute for free, but only from california. pingdom checks from all around the world.

Any of these online services will do the trick:

• Montastic
• Pingability
• Site24x7