I use Site24x7 for my website monitoring service – it’s nice because you can be notified via email and/or SMS whenever your site goes down. Sorry, it’s not free – I only use it because I signed up when they started and was grandfathered in. The website I’ve been working on has recently been having a lot of problems and I wanted to keep close tabs on it. This runs into a problem because Site24x7 only allows a maximum of 5 minute intervals for site requests.
A quick search on Google found HostTracker. It’s a handy web tool which does every Site24x7 can do (as far as I can tell). Better than that though, the 30 day trial allows up to 1 minute site request intervals…bingo! Very handy. The site is very easy to use and very helpful. I’d definitely recommend giving it a try.