Gmail Labs team added another useful feature, forgotten attachment detector. This is an experimental feature, you need to enable this feature from Gmail Labs tab. Go to Settings, Labs tab and enable this feature to use it.
After enabling this feature if you mention an attachment in your email but forgot to attach a file, it will give you a popup message while sending with out attaching a file.
There is one more new feature in Gmail, that is “mark as read” button, if you enable this feature you will see mark as read button beside delete button in gmail.
I created a video about how to use this lab. Check it out! http://googleappsfan.blogspot.com/2009/01/new-gmail-lab-forgotten-attachment.html