Android: Dropbox for Android is already a useful tool, but DropSync allows you to select a folder on your SD card and automatically have that folder synchronised to your Dropbox account whenever a change is made to it on your phone, or on another system.
When installed, you can tell DropSync to sync its contents with another folder in your Dropbox account or create a new folder for the app to sync with. By default, the app will sync the folders automatically. The free version of the app only syncs one folder, won't do subdirectories, and is ad-supported. That may be enough for synchronising a download folder with a desktop, but if you want subdirectories and multiple folders, the full version will set you back just under $US6 in the Android Market.