3DS Lib
The Nintendo 3DS Library.

Can I download 3DS Games / 3DS ROMs online?

YES - Games for 3DS and NDS can be downloaded form Nintendo RomSites.
3DS ROMsGame backup copies or 3DS ROMs are made for playing with R4 3DS flash cards and Nintendo 3DS Emulators. The best compatibility and speed can be achieved using a flash cart, but for those who just want to preview a game on PC before buying it I would suggest a 3DS Emulator. Also Nintendo DS / DSi emulators and nds roms are widely available for free download, and good R4 DS Flash Cards these days cost only around $20 USD, so there is absolutely no reason not to get one for your console if only to play 100% legal homebrew games and applications and enjoy MP3 / DivX / Avi and Comics.

Nintendo 3DS Roms
3DS ROM release list and files
3DS Backup flash cards - R4 3DS Gold, SDHC and RTS version.
3DS Flash Card
3DS Homebrew

Nintendo 3DS Hardware - R4 3DS

R4 3DS Card - Flash Cards and microSD adapters for N3DS roms R4i DSi R4DS SDHC.
Download R4 DS Roms also known as NDS Games for R4i.

3DS Tech Specs
3DS torrents P2P file sharing for games
3DS Hacked - latest news from the N3DS mod scene
NDS ROMs Download