Advanced Custom Fields Plugin for WordPress
Conductor Plugin: WordPress Content Types Without Code
Tuts+ Guide to Content Types and Relationships in WordPress
* This is an amazing 9-part series by Rachel McCollin that explains
content types in WordPress starting with the database itself.
It gets technical, but I highly recommend it!!
Custom Post Types UI Plugin: Create Post Types and Taxonomies
Smashing Magazine: The Complete Guide to Custom Post Types
Example Content Inventory/WP Content Types Spreadsheet