iOS: The default Dropbox client for the iPhone is a solid offering, but if you're looking for a little more functionality, Boxie brings all kinds of features to the table, including notifications, a powerful search and support for Dropbox versions.
Boxie has a few features that make it stand out from the official Dropbox app. Notably, the search bar allows you to refine your search to folders or subfolders so you can actually find the files you're looking for. You can also create shortcuts to specific files you access a lot, as well as store items for offline viewing.
The real handy feature in Boxie is the support for Dropbox versions. You can show all previous versions of a file in Dropbox and revert to older version right from inside the app. If you change around files a lot, this is a killer feature in itself. Boxie's packed with all kinds of features the Dropbox app doesn't have, so if you want a few more options, it's worth a look.