Assuming you on the road or something (or even asleep or something) - does anyone use a service to check their website uptime? To page or call your cell phone if your website goes down? So you can reboot your server or fix it more quickly?
Incredibly stupid question alert - Does it affect your webstats at all? Page views, etc? Or is it just trying to get a response code from your sites?
Forkqueue suggested monit earlier today: http://www.tildeslash.com/monit/ It runs on your own server... So I guess it won't work when the whole thing is down but if you are with a reliable host, how often will that happen? It does check individual programs running on your server and will check their integrity and status. You can access it via your mobile phone as well. It can send you e-mail when trouble is found - based on that I guess you can also make it call a web service that would send you a text/sms or call you.
I use http://www.watchmouse.com/ and their free OS X widget. Many of my sites are on the same server, so if my 'key' site goes down it's a good indicator that they're all down.
We've used Alertra (http://www.alertra.com/) for several years. They are not the cheapest in the world but very good with little false pos. We looked at hyperspin (http://www.hyperspin.com/) reciently and they looked good also.
I use monit simply to ensure all the services that should be running are running - and to automatically restart any that have failed (doesn't happen often, but it's nice to know it'll happen automatically). For uptime monitoring/alerts I use Nagios.
Hi. We can suggest you to try our free uptime monitoring service (URL in signature). Not only uptime monitoring but free SPAM blacklist checking so you will know when your site/server IP is blacklisted.
Yes. We have our own system doing it. We are actually using PageSentry on an old 6100 Mac along with PageNow. PageSentry monitors just about anything from a TCP/IP stream to HTTP, then sends out notifiers via email, page, cell, etc... I've got the system set up so that it not only emails me, but actually dials up and calls my pager, in the unlikely event email is not working, and sends text messages via another outside server to my cell. If it is down, we know about it before the client does. It's pretty ghetto, but it works! I've never used an outside service, but I would imagine there are plenty out there.