Chrome: Attaching files to Gmail from a cloud service can be tricky if you don't have that service synced on your current device. Cloud Attach is a Chrome extension that enables 'attachments' from Dropbox, Evernote, SkyDrive and Google Drive, amongst others.
To use the service, you have to set up an account with ZeroPC, the developers of the attachment, and link that account to the cloud-based services you use (as well as those already mentioned, there's support for Box and SugarSync, plus the ability to send pictures from Instagram or Flickr).
Once the extension is installed, you'll see a new cloud icon at the bottom of your Gmail compose window. Using this, you can attachments for any of the connected services.
Cloud Attach [Chrome Web Store]