We don't have a clear, localized source for all the tools for YI, so... Here is a list of currently available tools for Yoshi's Island, download links, and what they can do:
Name - Link - Purpose
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GOLDEN EGG v0.005 - Link - LEVEL DESIGN EDITOR, primary tool to mod YI
Ycompress - Link - Decompresses the graphics for editing in YY-CHR or other graphics editors
BTD6_maker's YI Text Editor - Link - Allows editing of all text boxes
YI Entrance Editor - Link - Edits level start points; do not use midring.exe, only yientrance.exe
Better Middle Rings - Link - Patch that makes it easier to edit Middle Ring restart points, requires asar to patch; recommended over the above for midrings
YI Sprite Set Editor - Link - Edits spritesets to use different combinations of sprites together
Yoshi's Island Level Tool - Link - Exports levels from and imports levels to ROMs
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Please feel free to post any others you find/create in this thread, and I will update this post as tools are posted.