Andorid: If you've been unhappy with the way Dropbox's mobile app only uploads files manually, you'll want to check out Titanium Media Sync, which will constantly sync folders from your phone to Dropbox, just like Dropbox does on the desktop.
From the makers of the amazing Titanium Backup, Titanium Meida Sync is what Dropbox's mobile app should have been. You pick folders from your SD card to sync back to your Dropbox. Any time you add something to those folders — say, if you take a picture on your phone, or download an app you want to try out on your computer later on — it will automatically sync those changes back, so your files will be waiting for you when you get back to your computer. You can also sync folders from your Dropbox to your SD card, but it's only a one-time sync — Titanium won't monitor your remote folders, only the ones on your SD card.
The interface is pretty easy to use, too. The left column shows all the folders on your SD card, while the column on the right shows your Dropbox, and you can just drag and drop folders from one to the other to sync them. Dragging from the the right column will download that folder to your SD card, while dragging from left to right will give you the option of setting up that folder for automatic sync. Because it only syncs one way, it won't constantly call the internet slow down your phone when it's waiting for changes. You can even set whether you want it to sync over Wi-Fi only, or over 3G and 4G in the preferences. By default, it will also sync only when your phone's plugged in to save battery (but you can change this in the preferences too). Seriously, if you're a Dropbox user, this app is absolutely worth having—even moreso than the official Dropbox app itself.
Titanium Media Sync is a $1.97 download for Android devices.