Gmail lets you view and save attachments directly to Google Drive from within your mail message.
Attachments are visible as previews at the bottom of a message. Clicking on the preview brings up a fullscreen view of the file, and you can navigate through several attachments the same way you flip through a photo slideshow.
Click on the Google Drive icon that appears when you hover over an attachment and choose where to send your attachment.
This is a great feature for those using the GMail app on an iPad or iPhone.