One of the lesser-known useful things about Gmail's filters is that they work on email you send as well as email you receive. So you can do things like label all messages you send with the word "Request" in the subject as "Waiting for," or label any email that has the word "Invoice" in it that you send to your client or boss with the "Invoice" label, or label any email that goes to certain addresses "Family" or "Softball team." There are some limitations with outgoing Gmail filters, however—you can't search on special characters (like [w] ) and you can't do certain actions, like forward the messages to another email address. Do you filter outgoing Gmail? What are your most useful filters? Share in the comments.