Ready-to-Use Database models example

David Hay's

• Enterprise Model Patterns. This is a beast of a book, but has some great patterns.
• Conventions of Thought. More stuff on MRP.
• A Meta-Data Map . Haven't read this one.

Len Silverston's

• Data Model Resource Book Vol. 1. Your main data model patterns.
• Data Model Resource Book Vol. 2. Case studies by industry.
• Data Model Resource Book Vol. 3. A deeper explanation of Vol 1.

Michael Blaha's Patterns of Data Modeling. This one has some interesting temporal, graph, and tree models.

Martin Fowler's Analysis Patterns. This one skims some of the other patterns, but gives accounting a solid treatment.

They are all well-rated, and I have read all but one, and they are all very good. Several of them are available on safaribooksonline.

Also, OASIS's Universal Business Language, schemas

• If it's more complex than the free data models you've mentioned, then I'd suggest it is a custom model for your requirements.

• Anything complex would most likely be commercial, proprietary, or otherwise someone's IP.
  For example, a data model for an MMORPG or a derivatives trading system.

What are you trying to do anyway?