This is an idea for an extension that could be coupled with any "quick open" extension (like Mozilla has) which creates an always-on icon in the task bar.
The idea is that when visiting a site the user could click an icon (or whatever) to add the page to the watch subscription list. The crawler would then periodically check to see if the page has been updated and then notify the user if it has been (by email, popup message, sound, &c).
The user would be able to select how often to check the page and how to be notified for each alert.
Obviously this is feature would be most useful for users with a permanent connection but even users with dialup connections might be able to use it to check daily whether favourite sites have been updated. Users with dialup connections could have more frequent watches -- perhaps even every 15 minutes -- for watching frequently updated sites (say news sites).
