Is there a webapp that will take a URL and scan it every few seconds or so for changes?
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If you want an OS X program, I believe this question belongs to superuser.com
In case you actually want a webapp (this website is about webapps), then I recommend ChangeDetection.com, I have been using it for a while, it works very well. Not sure about the check frequency, though.
Google Reader can do that out of the box. It can track changes even if the site does not provide a RSS feed.
Here is the blog post from the Google Reader team announcing that feature in January 2010.
Edit: Turning off the track changes feature as of October 2010.
I'm currently using Follow that Page
It allows you to check 100 pages/day and an additional 1 page/hour, if using a free account; Or 1000 pages/day + 20 pages/hour + 5 pages/10 minutes + 100 page/week if using a paid for premium account. (check the FAQ for details on how those limits work, also I'm still on the free tier.)
If a page has changed, it will send you an email containing those changes. Also, It supports areas to ignore while looking for changes, allowing you to ignore things like the amount of comments posted when looking for changes.
You seem to be asking for an webapp that checks a page every few seconds, be aware however that this is generally not appreciated by the website owner, as thi can lead to high bandwith costs and can result in you getting banned from the website you're checking.
I believe this Firefox addon does exactly what you are after: https://addons.mozilla.org/en-US/firefox/addon/4337/
This extension adds a small Delta icon to your browser status bar. When you click onto it, it highlights all changed text contents of the current page (since the last time you clicked onto it). Clicking again cycles through the changes. Rightclick onto the icon allows you to add scan or ignore regions and manage pages known to SiteDelta.
And regarding your wish to have it done automatically
Furthermore SiteDelta can regularly check its known pages for changes and inform you whenever one of them was updated.