I have been using Dropbox for some time now and it has proven to be extremely useful for my business. Basically, it is a free service (for up to 2GB of storage to start, but can be increased with referrals) that allows you to back up your files online, which then are synced with all of your computers with Dropbox. Or you can access your files online from any computer or even from an iPhone or Android phone!
For example, you can work in a QuickBooks file remotely on a laptop and when you are finished, it will automatically sync your changes so that when you get back to your office, the QuickBooks file you worked on is available on your PC. You can also share folders on your computer to others so that they can drop files into it that they want to share with you. I used to use services like YouSendIt (free version), but Dropbox is better because there is no limit to the size of the file you want to share or how long it is accessible. It works just like a shared folder on a wired network. Therefore, the need to use a thumb drive or flash drive has become obsolete.
For more strict permissions, like view-only, on files, I complement this service with Google Docs. However, the amount of free storage space is only 1GB, so I only use it for these and other special circumstances. But Dropbox works best for backups, file sharing, file syncing between computers, and web and mobile device access.