I find it very undescriptive when I recieve a message from Drupal with the subject "message from username".
This patch adds a "subject" field for the contact.module which the user can fill, and would tell you what they want right away.
Oh, and it helps group the 'conversation' on Gmail into something meaningful.
(Note, I have not tested this since I do not have a CVS installation at the moment. Appreciate if someone can test it).