I've written about ExpanDrive in the past, but I haven't mentioned it since version 2.0. Version 4 is out now, and it's a major update to this app that allows you to mount remote servers and cloud services as local drives.
ExpanDrive has always done an amazing job of mounting a wide array of remote server types and letting you use them like your server was just a local USB drive. Lately, the number of accessible services has become wider than anything else I've seen, and the usability of the mounted drives is outstanding.
ExpanDrive 4's biggest advancement is in speed. It can now cache files locally and transfer smaller files in the background, meaning you can mount a remote server, edit files in a text or graphics editor, and when you hit save you're immediately able to continue working while the actual transfer happens transparently. It can even watch for changes on the remote server to keep the sync rapid and up-to-date on both ends.
ExpanDrive has also been adding to its available services. You can now mount everything from SFTP and WebDAV servers to Dropbox, Box, Google Drive, Amazon S3, and DreamObjects cloud services. The latest services include Microsoft OneDrive, Copy.com, HP Helion Cloud, Owncloud and hubiC.
Being able to mount a Dropbox drive without installing the Dropbox app means easy access to your synced files on remote computers without the bandwidth of a full download, or even the trouble of setting up selective sync. This holds true for any of the supported cloud services. It also means easy backups to the services without any special software.
ExpanDrive has a free trial available (and is also available for Windows). You can license a copy for $49.95 US.