Montastic is an extremely handy tool that monitors websites for you and lets you know when they go down.

This is great for developers who need to keep track of numerous sites (up to 100). Montastic checks your sites every 10 minutes and as soon as one or more sites go down, it will instantly send an email to you and up to five other people. Best of all, the service is completely free.

If you want something a bit more detailed, mon.itor.us offers more detailed stats and charts and allows you to monitor, amongst others, HTTP, FTP, Ping and POP. This service is also free

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