In Gmail, under the Subject box there is a link to insert an invitation. Clicking this link will open a pop-up that will let you create a calendar invitation. Everyone you send the email to will receive the email as a Calendar invitation. They will be able to reply and add the event directly to their calendar. While the pop-up does not give you all the functionality that you can choose when making an event directly in Google Calendar, it works wonderfully for quick or single events. See the attached screen shots for a quick over view of this process.
Below is the pop-up window you will see. Notice how the Subject heading of the email becomes the "what" heading for the event.
