Dropbox has released a new extension for Chrome that adds a nice new feature to Gmail. It's call Dropbox for Gmail. It allows you to more easily share files within Gmail. You can quickly add a link to a Dropbox file from within Gmail, just like you can link to a Google Drive file. There will be a Dropbox icon in Gmail's compose window that you click to browse and select files.
This is a great resource for people using Gmail and Dropbox. Check it out on the Chrome web store.
Dropbox adds new features - document preview and photo sharing
Lots of Great File Sync/Backup/Share services