Homebrew:

PieChart: Wiibrew Page. Local download (0.8) here.

SADX:

Sonic Adventure RDX
X-Hax's flagship project. A complete retooling of Sonic Adventure DX.

Endri's Super Sonic Hack
Latches onto a sonic.exe and applies a series of code fixes to allow playing as super sonic in levels. (Direct download only - no web page).

No Gloss (heroes)
Takes the specularity away from the character models of the Sonic Heroes Demo. (Direct download only.)

No Gloss
Takes the specularity away from the character models of SADX. (Direct download only.)

Dreamcast Model Ports
project that aims to port dreamcast player models to SADXPC

xbox 360 controller patch
Quick, dirty project that fixes the 'rotating camera' bug when using certain controller setups. (Direct download only.)

MAXScript Nodetable Exporter, version 1.5
This tool lets you create a level in 3ds max and export it in a format that SADXLVL can handle.