Kim Dotcom has always said that he plans a powerful API for Mega. The API is not here yet and probably won't be for a while, but that doesn't mean there aren't developers interested in using the service.
Granted, some aren't exactly to Dotcom's liking, Mega Search for example, but others are probably more welcomed, like the first ever Android app.
Mega Manager Alpha enables you to sign into your Mega account, download files and that's pretty much it. As the name implies, this is an alpha release, bugs are common and features are lacking.
Still, if you need a way of getting files out of Mega and onto your phone and don't mind the occasional crash, the app might be what you need. You can download multiple files at a time and everything goes into the MegaManager folder on your SD card.
More features are coming though, not to mention bug fixes. On the to-do list there are opening files, uploading, file management and many other features.