Once upon a time I wrote a desktop application named ContentSpy. It let you monitor web pages for changes, and notified you as updates became available. When I recently heard of the new Google App Engine being launched, I figured I would take a stab at porting ContentSpy to that platform. I am happy to report that I stopped my endeavors, after stumbling upon ChangeDetection, which offers exactly what I envisioned, for free!
Another good thing coming out of this discovery is that it won't distract my focus from working on FeedJournal...
According to Alexa, ChangeDetection has been online since 02-Nov-1999. On the other hand, their Alexa history is only available since last summer. Wayback Machine doesn't have any historical records either. Technorati's oldest indexed post referencing ChangeDetection is from December 2007, so I assume it's a new service. Besides, I can't imagine it having escaped me for that long!
Anyway, ChangeDetection is wonderful in many ways! It sports a simple Google-like interface and sign-up procedure, and offers all the relevant features. I currently need this service because I want to buy a mobile phone from the local operator and expect prices to drop soon. So I simply tell ChangeDetection to monitor the price list page for me. When a change becomes available I am notified by e-mail with the actual change highlighted.
Another site I added for monitoring is IBM's "Ponder This" challenge. Each month IBM publishes a new brain teaser for the community to solve. Amazingly enough they don't offer any way of subscribing to new challenges, but ChangeDetection comes to the rescue once more.
I don't see RSS feeds and ChangeDetection overlapping in any way. For my private use, they are perfectly complementary. All sites have pages not covered by RSS feeds, and there will always remain sites that lack feeds completely. ChangeDetection is optimally suited for monitoring modifications on these pages.
I welcome you to add comments to this post about pages of public interest that you have chosen to monitor with this service. Please, go check it out!