Are there any services that will monitor a web site and report if it goes down, or experiences unreasonable response time? Which are reputable and recommended? How does the service notify you when your site is having problems?
aremysitesup.com is an easy solution, with a free option. I use it for my personal blog.
I would recommend http://www.pingdom.com.
A more server level service is http://www.serverdensity.com/. Which requires you to add a script to server, but will give you real-time graphs or MySQL, Apache etc. Server Density also allows you to monitor HTTP and TCP for that you won't need to install anything at all and you'll still get graphs and alerts for the monitoring of websites.
I use UptimeRobot and have been very pleased. It's pretty basic monitoring (does an URL that you specify respond with 200OK and in a reasonable amount of time?), but it's free, and they offer alerts via email, SMS (via email gateways, which are sometimes delayed), and Twitter.
I recently had an outage while I happened to be using the server, and I was notified within 4-5 minutes. I'm definitely pleased with the service, considering what I'm paying for it. ($0)
http://www.alertfox.com has free and paid accounts. What I like about this service is that it offers transaction monitoring in real web browsers (IE, Firefox and Chrome). We use it to monitor our website including the Flash games.
Server Mojo it's not free, but they have a continuous going on offer for 30 bucks / year for monitoring 1 site/domain.
The monitoring is excellent because they don't only check for the server header 200 success, but you can also check for a keyword whitin your page, in this way it tests each time the server uptime and also if the page was really given out completely and if the PHP engine is running.
BTW: i love their leitmotiv: "RED is BAD!"
UPDATE March/2012: STAY AWAY from Server Mojo!!! After trying their service for almonst one year (i paied), I discovered they have no support at all, I received false downtimes signals, and now looking also at their forum (last updated in April/2011) I think they are dieing or something anyway is not working out.
Server-side monitoring software:
Site monitoring services:
Remote monitoring/testing software: