I've been working on a GUI library for the Nintendo DS (called "Woopsi" - spot the pun), with the hope that other homebrew authors will use it instead of wasting their time writing half-baked GUIs of their own. As an Amiga guy, I decided to go for a Workbench feel to it, so it might be of interest to people here.
Anyhoo, I've attached a screenshot of where I've got to so far. Current features include multiple screen support, screen dragging, windows, gadgets, a text viewer, Blitz Basic-style SuperBitmaps, screen backgrounds, an event system, and more.
If you want to have a look at the latest demo, you can download it from the SourceForge project page here (download the demo package; the other package is the sourcecode):
The zip file has the usual ROMs for the SuperCard, Slot-1 carts and the nds.gba file. If necessary, patch the .nds file for your card.
If you don't have a card but want to have a look anyway, you can use the DeSmuME emulator:
Finally, if you're interested in learning more about the system, there's loads of info on my blog: