Pig Sentry is for real-time alerts, without getting spammed. It is intended for a high volume of alerts (the site it was implemented for, sees 200,000 to 300,000 alerts a day).

The way Pig Sentry works is it maintains a state table of recent alerts. It will send a notice if there is a new alert, or if there is an increase in the general trend or pattern of existing alerts. The trend notification is fairly configurable. It also expires alerts after there has been no activity with them, for a while.

Pig Sentry also checkpoints its state information to a file periodically and when it exits, and this same state store file is loaded when it starts up, so it can easilly be rolled into a lot rotation mechanism. The store file is not horribly readable, but is somewhat useful to peek at if you are interested to know the current profile of alerts on your site.

Homepage: http://solv.com/tools/pigsentry/