I'm looking for a service that will monitor a page, e.g. a classified site, on a regular interval (around one hour) and email me the changes on that page.
If the website does provide an RSS feed skip step one and go directly to the second step. Look out for the icon on the website or your browser. Just click on it and use the URL you get at step two (Firefox shows the icon by default but not from version 4 on (you need to get it from the advanced menu), IE shows it from version 7 on (it has a star if the website has a feed) and on Chrome you need an extension).
Go to one of the following websites which convert websites into feeds. None of them provide an one hour update interval on the free plan but most allow upgrading for a few if you need it (you can try the free plan to see if the app is reliable and you like the format which largely depends on the site you monitor - all give different quality for different kind of websites, there is no one size fits them all solution):
I haven't used all of them but it doesn't seem possible to give one perfect web app. It just largely depends on the site you want to monitor. Just try to find the one that suites you.
Use a service that mails you updates of the feed URL you got from the first step:
- feedmyinbox (free only for daily updates)
- Blog Alert (daily only)
- RSS to Email Gateway (doesn't provide information on how often you receive updates)
You can use any of these but I would prefer feedmailer as it worked perfectly as I last tried it and you can set the frequency you need. If you don't need to rely on an email you can uses a software like Google Reader which is much more reliable. It gets you the update of the feed of step one in an web interface.
The site does not have an RSS feed I take it? You can add the URL to Google Reader AFAIK and it would make an RSS feed of the page and notify you of changes.
Found Change Detection on Google - never used it though
I have been using Watchthatpage with satisfactory results over a long time now. They make sure they email for every minor change in the site's html/css.