DownloadMii is an online marketplace for Nintendo 3DS homebrew applications and its 100% free of charge for the end-user.
It is currently under development, and the GitHub project can be found here! You can read about the latest news, 3DS news and app releases/showcases here.
To use DownloadMii, you will need a way to run .3dsx files on your Nintendo 3DS.
The .cia format for the main application in the future, However DownloadMii will still only support downloading .3dsx applications.