Activity Monitor is AWESOME at letting me know when my site is being brute forced. I can see that these addresses are using a dictionary attack, and that they hit once an hour (average) and switch IP addresses every day or so. Without Activity Monitor, I would never have known that this was happening.
But what about taking it to the next level? How about an IP ban function on the same screen, or an icon beside their IP? What about an auto-ban for IPs that fail authentication 5 times in a row? Now THAT would make a hell of a plugin :D