Dropbox offers a way to synchronise files between all the computers you own. It also enables you to share folders with other dropbox users - great for families, students, and project managers alike.
The first computer
The second computer (and third, and fourth, and fifth, and....)
Inviting others to the party
This feature can also be handy if you want to share folders full of photos with family members etc...
The bad bits?
The only other disadvantage is that it is currently only available for Intel/AMD based systems. A lot of network servers and newer portable devices are based on PowerPC and ARM chipsets. While this isn't a problem for your average user, it can make the decision of weather to use dropbox in a corporate environment or not a tricky one. The dropbox team have said they are working on PowerPC and ARM versions of their client, but so far nothing has materialized.
On the other hand, there are apps for accessing your dropbox files from iPhone and Android based devices.
You can sign up for a dropbox account and join in the fun here.
Dropbox is an excellent piece of technology. Because of it, I rarely carry a usb drive around with me. There are a few issues with the available packages and support for embedded devices, but that is a business issue and should not be a problem for most casual users.