Free Online Site Monitoring
I host my web server out of my house, but regardless, I really need to know if there are any problems with it. Hosting out of my house means that I cannot use my home server to do the monitoring or even another device at my house. This presented me with needing to find a remote monitoring option. Fortunately, recently I have seen quite a few options being released in this area with a complete enough feature set to get my basic needs. Ideally, I am waiting for one of these to offer more port monitoring but I don't pay for these so I can't complain too much.
Pingability
I used Pingability for a long time to monitor my site. It is free and offers at best a ping once an hour. At the time, this was better than I had but this is not really fast enough. I have also found that their monitoring isn't 100% accurate as I have often done maintenance and not been notified of an outage. To the left is an example of the daily report they send. Sometimes I would receive a report that only monitored for 16 hours out of a day. Another complaint I had is that they only did a ping monitor, rather than an HTTP service check or a response time check. I stopped using this service around 2/1/2011.
Uptime Robot
I recently switched to Uptime Robot and I find their interface to be very slick. It is all CSS/Javascript driven and very smooth. I setup two monitors on this service, one to ping my server and another to check my webmail port and do a keyword check to make sure that apache is responding. They will charge you to send SMS messages but do not charge for email. Fortunately, most cellular providers provide an email interface to send an email to text your phone. I setup both monitors to email my phone whenever an outage occurred. This weekend, my web service stopped responding while the server and email itself all remained working. Had I been using Pingability, I would have failed to be alerted but fortunately, I was able to begin troubleshooting right away from my cell phone. So far, I highly recommend this company.
UPDATED (4/4/2011) - They recently added a port monitoring service that allows you to monitor any given TCP port on your server. They have a limit of 50 monitors so this combination is extremely powerful. I am now able to monitor my 3 email ports, DNS, MySQL, in addition to an ICMP monitor and a keyword check on one of my websites. This is all with a 5 minute monitor interval, which is just incredible. The new interface continues the slick use of CSS that they previously had to give a lightweight but powerful interface.
Monastic
Montastic offers URL checking at most every 30 minutes and can check for the presence or absence of keywords in the http output just as Uptime Robot does. One nice feature of Montastic is that it supports Google, Yahoo, and OpenID accounts so that you don't have to sign up with another account. Setup takes less than a minute. Like Uptime Robot, you will need to setup email to your phone if you want to be texted when your service goes down. I am testing this service out and can recommend it, but being that Uptime Robot provides 5 minute checks, I would recommend them.
FreeSiteStatus
FreeSiteStatus.com offers multiple protocol checks such as DNS, MYSQL, POP3, or any TCP port. They have 17 servers world wide to perform checks from. The interface isn't as pretty as Uptime Robot but it has some nice automation. When you setup a server, it will scan your server and offer you to setup monitors on all of the open ports it finds. Monitoring is free but limitted to 5 services before you have to buy more. Alerting to email is free but SMS is not. You have 2 for free and then you have to purchase additional alerts. It offers monitoring as often as one minute on all services. Overall, this is another service I would recommend you to check out.
Binary Canary
Another monitoring service with a slick interface is Binary Canary. As the name suggests, it is the digital equivalent of a canary in a coal mine. They have basic monitoring offered at 15 minute intervals. The interface is a bit more complicated than it needs to be have you setup monitors in a completely different section than the servers themselves but nonetheless it works. The online reports look great and use Adobe Fusion for a nice presentation. It gives you your uptime, the date of your last failure and the duration of that. The free plan lets you monitor up to 5 things among any number of devices. I recommend checking this site out as their product is at least compelling.
Monitor.us
This is one that was recommended to me by some coworkers. The free version allows you to poll every 30 minutes and send alerts to an email address but not an SMS device. Fortunately, many phones have an email to sms gateway so you can get paged if your service goes down. They support monitoring of most protocols, including HTTP, HTTPS, IMAP, SMTP, MYSQL, and FTP. A very cool feature it supports is internal monitoring where it can connect to an agent on your server and monitor utilization of memory, cpu, and storage and page you if needed. The interface is very dashboard like and you can move around the monitors on the screen to position them however you like.
There are obviously other services but these seemed to float to the top of my searches and I have used all of them. If you find another one you want listed, feel free to drop me an email with the info and I'll post it right away.