In a nutshell, Dropbox smartly and automatically synchronizes a hierarchy of folders among any number of Windows PCs, Macs, Linux PCs and iPhones.  All of the synchronized changes are automatically backed up on Dropbox’s servers, and you can go back into the past to retrieve previous versions or deleted files.

Here’s how it works:

1. Sign up for a Dropbox account, which comes with 2GB of storage for free (you can pay small monthly fees for more storage).
2. Install and configure (i.e., enter your email address and password) the Dropbox client on two or more computers or iPhones.
3. The client creates a Dropbox folder whose contents you can manipulate like any other folder on your computer.
4. Any changes you make to the folder on any of the clients are automatically propagated to all of the other computers linked to your account.
5. The clients are bandwidth-sensitive (e.g., you can install Dropbox on your laptop and it won’t take up all of your 3G bandwidth when you’re tethered to your phone) and supposedly even know how to synchronize between clients on the same LAN.
6. The files you put into Dropbox are also accessible through their Web site, and you can share links to specific Dropbox folders to friends or colleagues.

As noted above, you can get 2GB of storage for free.  Furthermore, you can easily get a 250MB bonus just by spending a few minutes using the service (for details, click on “Getting Started” from the Web site home page after logging in).  You can also get a 250MB bonus by signing up through another user’s referral link (in case you haven’t noticed, all of the links to the service I’ve posted in this blog posting are referral links for my account ) and additional bonuses for referring other people.

There are other services like this one, e.g., SugarSync, but this is the best one I’ve seen, and the fact that it has Linux support is totally sweet.  My only complaint is that it doesn’t (yet) have a BlackBerry client.