Super Mario 64 DS Editor
The level editor for Super Mario 64 DS.

Should be simple enough to use, but if in doubt, ask on the forums.
Of course, that's no excuse to not read the readme. That file was named that way for a reason, y'know.
Ver. Description  
v2.0 beta3 ROMs patched by it can finally be run on R4/acekard2i flashcarts. Also, better collision in imported levels. Download
v2.0 beta2 Fixes a few bad bugs spotted early in the previous release. Download
v2.0 beta1 The key features of this release are graphical rendering for many objects, and model importing.
However, the latter is experimental. Use at your own risk.
The editor was also rewritten in C#, which is a much better language than C++/CLI.
v1.01 Same as v1.0, but compatible with old video drivers. Download
v1.0 Actually usable. Equipped with a 3D preview of the level being edited, and a decent editing interface. Download
v0.5 Officially numbered v0.5, but actually versionless. Very simple (yet functional) editor with no graphical preview.
Use not recommended. C'mon, there's way better than that now.