TitsUp
TitsUp is a simple service that will check any URL to see if it’s responding correctly. Enter either a domain name or full URL and the site performs a number of checks including DNS resolution, connection, HTTP request and the response.
If anything fails it will tell you, in detail if required, and offer to monitor the URL for the next 24 hours, emailing you when it becomes available. It can also notify you via Twitter.
The most common use for this service is to check if a website that appears to be down is actually down or if it’s just your connection to the Internet that’s having issues.