SMW Central is the main website for all of your YI (and SMW) romhacking needs. They host a large library of completed hacks, as well as everything you'll need to create your own! Keep in mind that not every YI hack that exists is SMW Central (e.g. single level hacks). We'll try our best to do that here:
- SMWC Yoshi's Island romhack list 
- Level design contest 1 
- Level design contest 2 
- Level design contest 3 
- Miscellaneous hacks 
- Boss Rush 
Since it's not legal to share ROMs, you'll have to get one elsewhere. However, sharing patches is perfectly legal, as they only contain the changes made to the game (custom levels, assets, code, etc.). Patches can be applied online here  or locally with the program Floating IPS (Flips)  - either method is fine though.
It's recommended that you use the web patcher because it's easier and more convenient.
1. Select an unmodified 1.0 (U) ROM of Yoshi's Island.
2. Select the patch you want to use.
3. Rename the ROM (if you want to).
4. Click "Patch".
Video tutorial 
1. Select "Apply Patch".
2. Select the patch that you want to use.
3. Select an unmodified 1.0 (U) ROM of Yoshi's Island.
4. Click "Save" (or rename the ROM if you want to).