It’s crucial for a website to experience minimal downtime (and for site owner to monitor uptime), otherwise visitors or customers won’t return again if the site is always offline and inaccessible. Sometimes it happens that websites are going down even if you have a very reliable hosting with 99% of uptime. The reasons could vary: problems with data center, network outages, server outages, server overload, DNS configuration, or SSL certificate problems. In this case the problem has to be solved as soon as possible.
Website uptime monitoring service will notify you whenever the website or server is down or having access problem. The basic idea of uptime monitoring services: they run in the background checking your website uptime through different ways, protocols and from different locations.

If something goes wrong and your site didn’t respond, you get notification by email or SMS. With the use of uptime monitoring services you can check claims of your hosing provider about 99.9% of uptime. Advanced monitoring services also lets you check content of you website in place (site isn’t hacked or domain stolen, etc.), SSL certificates and other useful features. If you know any good free uptime monitoring service not mentioned here, please add it in the comments.

Basic State
Basic State is a free web site uptime monitor service which checks your website every 15 minutes. You can get instant trouble alerts by email or SMS. It lets you check unlimited number of websites.The uptime checks are made by multiple ways: DNS, network and server response. Using Basic State, you can choose to receive additional server alerts for the external ecommerce services which might affect the speed, availability or performance of your web site. These include credit card payment processing gateways, affiliate networks and advertising networks.

mon.itor.us
mon.itor.us provides free network and website monitoring service with personalized interactive interface, where you can add server performance and availability tests, get uptime reports, add contacts to get the alert notifications. Tests will be performed from geographically distributed servers. mon.itor.us offers monitoring for different web protocols. It is very easy to set up; most of the services are free. It can send weekly reports or only notify you when there is a problem on your network.

InternetSeer
InternetSeer claims to be the world’s largest website monitoring service with over 1.3 million user base. InternetSeer monitoring systems remotely check your website from several geographic monitoring stations at selected intervals. If the monitoring system is unable to reach the site, an email, cell phone or pager alert is sent to notify you of the problem. Some of the key web site monitoring services available include: availability monitoring, performance monitoring, link and image checking, transaction monitoring, historical and detailed check and alert reporting.

Montastic
Montastic developers say that it is the “free website monitoring service that doesn’t suck”. With Montastic website monitoring tool you get an email when your site goes down, get an email when it goes back up. You can read statuses via RSS or Mac & PC widgets. You can monitor Up to 100 websites. Montastic has support for https and port number. This uptime monitoring tool checks your website at least every 10 minutes from 2 different locations.

Uptime Robot
Uptime Robot monitors your websites every 5 minutes (which is really frequent monitoring), totally free. You can add up to 50 websites. You will get alerts if your site is down by e-mail, SMS or RSS (Twitter coming soon).

SiteUptime
SiteUptime is a website monitoring service that checks your website at regular intervals and notifies you via email or SMS if more than one location detects a connection failure. In free version you can monitor only 1 website every 30 or 60 minutes. SiteUptime also sends you free uptime report monthly. Monitoring options include: HTTP, POP3, SMTP, FTP, SSL, DNS, custom TCP Ports, ping, HTTP Password Protection.

Are My Sites Up?
Are My Sites Up? free plan will check up to five of your sites at least 25 times per day. You get unlimited email and SMS notifications containing also the reason why your site was down (HTML status error code). Even if their website goes down, developers promise: “If you aren’t able to load the site, don’t worry! Your sites are still being checked in the background and you will still be notified.”

Uptime Dog
Uptime Dog is a free website monitoring service which will check availability of your website every 2 minutes from two different geographical locations to see if it is online and working, and notifies you to your email address if it is down. The only drawback of this service: you need to place HTML link to their site.

Root Internet
Root Internet offers multi-protocol support, uptime and performance of your website or server, downtime notifications via email and SMS, global monitoring locations. With “Live Status Report” widget you easily can display an uptime report on your website. You can define (one-time or recurring) maintenance windows so that Root Internet pauses monitoring during those periods or simply exclude them from uptime reporting. Root Internet can import the uptime history into your account for free, making the switch easy and painless.

LogicReach
LogicReach is another free web site monitoring service. In the case your site is running very slow or down they can send you an email or SMS. In addition to the monitoring, LogicReach also tests response time – page load time and offer extensive reporting facilities to allow you to monitor your site effectively.