Astro File Manager
Staying productive on your mobile device often involves mucking around in the Android file system, placing and plucking files from here, there, and everywhere. That file system is hidden from users by default. Astro lets you dive in. 'Nuff said.
Actually, not quite. Astro also has killer search capabilities that can find your files no matter where they're hidden on your phone, and "Cloud Hopping" technology that lets you seamlessly move files between multiple cloud storage services without downloading them to your phone first.