The point of Moonshell is to view media files, whether they be mp3 audio files or image files or text files or converted video files. It's basically the de-facto media player for the DS, even though there are dedicated apps that can handle some of those individual functions better (DSReader or DSLibris for text files, ComicBookDS for image files).
There's tons of other homebrew out there, so much that we can't really give decent recommendations without knowing what types of things you're interested in. You can use your DS to take notes in class (DSNotes), calculate your tip (WabbitDS which emulates the TI-83+ calculator), play some retro games (NES DS, Lameboy, jEnesisDS, SNEmulDS, MarcaDS), stream internet radio stations, browse the web, message your friends through IM or IRC, control your computer remotely, talk to another DS like a walkie-talkie using the DS's local wireless capabilities, or even make a phone call over wifi(SVSIP). Those are just some basic suggestions. There are also tons of games people have made, from 3D racers (Whee!DS, Futuristic Racer) to sandbox simulators (World Of Sand) to a Portal clone (StillAliveDS) to rhythm games (Video Game Hero, AmplituDS) to guitar tuners to platformers to puzzle games to Tetris Attack clones (TetAttDS) to... well you get the point. Point is, the DS can do a lot of stuff if you look around to find the apps for it.