I use montastic for a few sites and find that it works pretty well given that it's completely free for 100 sites at a time. If you are a "roll your own" kind of person and have a persistent net connection somewhere with a Unix/Linux box, you can set up a simple cron job to do this for you. I've toyed with that before, but never really took it far enough that I was happy with my version vs. something like montastic. Siteuptime's uptime graphs seem pretty nice, though I'm not sure if it's worth the extra funds.
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