But when free gets you something useful without gotchas, then free is excellent.
Uptime is simple.
You then provide the URL of that file to Uptime, along with an e-mail address that should be notified when your server becomes unavailable. Uptime also creates a log page that you can elect to keep password-protected so that the log remains private. This is a neat, effective service that many sites will find useful. And the price is right. He writes the weekly Backspin and Gearhead columns in Network World.