Services or tools that monitor hosting Service Level Agreements

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Anyone know of a service or tools for monitoring the service level provided
by one's web-hosting company? I'm thinking of something that will provide
info like:

 - Uptime statistics (broken down by day/time)
 - Page download times (broken down by page/day/time)

I'm interested in monitoring the sites of small-medium sized businesses, so
need solutions that are in that price-range.

Obviously the use of a tool risks combining one's web-hosting company's
problems with one's DSL provider's problems. However, triangulation could
probably sort that out.

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