On May 18th, Dropbox will no longer support the OS X Tiger 10.4 and OS X Leopard 10.5 operating systems. 

Since one or more of your computers is running the Dropbox desktop application (client) on an older operating system (OS X Tiger 10.4 or OS X Leopard 10.5), you'll need to update to OS X Snow Leopard 10.6 or later to continue to access your files through the Dropbox desktop application. Apple's instructions on how to update your operating system can be found here.

If you're not sure which of your computers is running OS X Tiger 10.4 or OS X Leopard 10.5, you can visit your Dropbox account settings here to view all of your Dropbox-connected devices.

If you don't want to or can't update your operating system, your files will still be available through the Dropbox website. However, on May 18th you'll be signed out of your Dropbox application on your computer and the Dropbox desktop application will no longer be accessible.