There is a site that I need to monitor for changes and pop an alarm if the change is over a certain threshold. Ideally it would use Growl to pop the notification but I'm open to alternatives.
If there is an application out there that does this for purchase, I'll just buy it. If not, pointers to any Linux / Java / other recompilables that I could just add Growl support to would be very helpful!
If I get your question correctly you're looking for an application that gives you a notification when a given web site changes. If I'm wrong, ignore this answer.
The widely accepted solution to this problem is RSS. Googling led me to an RSS Growler which might fulfill your needs.
If the site you want notifications for doesn't support RSS there isn't an easy way to monitor it other than polling it.
That would depend on what you define as the "change is over a certain threshold". I've used Firefox's Check4Change to monitor for website changes effectively in the past, which allows you specify which portion of the web page you'd like to monitor, and how frequently it should monitor the page. An additional feature includes sound or popup notifications.
