Growl
Growl is a popular and common program for the common management of notifications from various applications.
If you have Growl installed then Maildrop will use it to popup notifications that Email's have been added to Salesforce, or that Case's have been created.
This is particularly useful if you've turned off the "show newly created XXX" option in the preferences.
You can disable the Growl notifications through the Growl preferences (just like notifications from any other application using Growl)