There are a few information web sites I need to monitor for changes,
and I have been looking for monitoring services without much luck.  I
need something that allows me to automatically scan for updates at
least every 15 minutes (or quicker), but so far the most frequent I've
been able to locate offers hourly checks.  Are there any services out
there that give me the freedom to scan as often as I need?

UptimeTest.com
Remotely monitors sites or servers.
Alerts by SMS, e-mail. Free Trial.
www.uptimetest.com
	
Website Monitoring Tool
Monitor your websites functionality
Is it working as intended ?
www.kapowtech.com
	
Watch Web Pages
Get instant alerts. See what's
changed. Download Free!
URLyWarning.net
	
Web Monitor $9.95/month
Monitor, detect, measure and alert
urls and transactions. 24 X 7
www.techout.com

http://www.pluck.com

This free program will download news feeds, but also will monitor web pages.
You can set the update time. I was able to set it to every 15 min.

For those who care, I've monkeyed around with a bunch of these now and
by far my favorite is URLyWarning.  My only reservation is that it is
software installed on my machine as opposed to a web-based solution,
but the end product is what I'm looking for.