Chrome: Dropbox released the beta for its new Chrome extension, Dropbox for Gmail today. As the name suggests, the extension makes it easy to instantly add a link to a Dropbox file directly from the Gmail web app.
When you compose a message, you'll see a Dropbox icon at the bottom. Just click that, select the files you want to include from your Dropbox account, and they will get shared with the recipient. It's simple, but gets the job done in a few less clicks. It seems to be in beta at the moment, but we didn't have any problems with it.