My product list page is showing unexpected products (or none at all)

This can happen from time-to-time, especially during development. Try following these steps in order. One of them will typically resolve the issue:

1. Make sure you have products assigned to the category (either search-based, or manually assigned)
2. If you've turned off the "Product Page" tab, make sure the "Product Selection Method" field (turn on "Standard Fields" in View ribbon) is set to "Manually Picked Products"
3. Check that nothing is stuck in workflow
4. Clear the Sitecore cache (/sitecore/admin/cache.aspx)
5. Perform a smart site publish
6. Rebuild the links database
7. Rebuild search indexes
8. App pool recycle (for good measure!)