In my Network World Web Applications Alert newsletter I recently covered a terrific service (in beta, natch) called Dropbox published by Evenflow.

Dropbox is a cross-platform, multiple-endpoint, file synchronization system. It creates a special folder called "Dropbox" in the local file system of a group of PCs. Drop a file into this folder on one machine and it appears in all of the other machines' Dropbox folders. This facility is incredibly useful for collaborative projects and the distribution of specialized content such as templates.

The actual file transfer is done blockwise (that is, using only file deltas) in the background to minimize perceived performance impact, and the actual transfer is mediated by the Dropbox servers which allows synchronization between machines isolated by firewalls because Dropbox only uses ports 80 and 443 (that's HTTP and HTTPS, respectively). An important feature is that all data transfers are secure and storage on the Dropbox servers is encrypted with 256-bit AES.

What's really impressive about Dropbox is you can install it on Windows (XP and Vista are supported, although it seems to work just fine on Windows 2003 SE) and OS X (Tiger and Leopard). Even better, a Linux version is in the works.

You can create subfolders under the Dropbox folder and share them with other Dropbox users outside of your group, and the Dropbox Web interface provides control of sharing, a log of all file additions and deletions, and recovery of deleted files; and you can add comments to shared folders. Finally, as if all that weren't enough, you can just log in directly to the Dropbox Web site and upload and download files, which provides you with access even from machines that don't have Dropbox installed.

During the beta (drop me a note to gearhead@gibbs.com with the subject "dropbox" and I'll send you an invite) you are limited to 2GB of shared storage. When Dropbox comes out of beta the company plans to reduce that to 1GB for free accounts (beta users get to keep their 2GB limit). In their FAQ the company notes that there have been requests for self hosting and to use Amazon Simple Storage Service (Amazon S3) as the back end, but it makes no promises as to when it might deliver these features.

It's got to be said: Dropbox is a work of genius. It does what it claims to do better than any other service I've seen and transparently enough that you could deploy it to inexperienced users.