Windows/Mac/Linux: Dropbox has quietly offered up a release candidate version of an upcoming 1.0 release. There are lots of bug fixes and performance improvements, naturally, but the RC includes "selective sync", letting you control which folders are synced to your hard drive.
In other words, if you're using multiple machines and you don't need every folder in Dropbox stashed on every system, you can uncheck certain folders in your Dropbox settings to keep them stashed only on Dropbox's servers. They're still there if you need them, but if you've got a sizeable Dropbox space, you can save yourself some hard drive sectors on your smaller machines.
The 1.0 RC is a free download for Windows, Mac and Linux systems. If you're currently running version 0.7.110 on your Mac or Linux system (mouse over the system tray/menu bar icon to see your version), don't upgrade directly to this release candidate without reading Dropbox's tips at the link below.