3DNUS is a Windows program that allows you to download NUS content for the Nintendo 3DS and convert it into .cia files if desired. The application is simple and easy to use. It's light, and works quickly.

Note: Nintendo has recently been able to patch the way 3DS' grab titles from the CDN and otherwise. This takes effect for firmwares 11.8+. As a result, it is possible that Nintendo will discontinue the ability to download content from CDNs that are not intended for use on 3DS devices. Meaning, 3DNUS will most likely become obsolete. If you need to download any firmwares or titles; it is advised that you get them soon/while they are still available.