This is documented in one of the readme files that ships with CRM 3.0, but I thought I would make note of it anyway, should you not be the readme type of person.

CRM 3.0 ships with an ASPX page that will list then entire CRM schema and allow you to browse through the Entities and their Attributes and Relationships, without loading the CRM Web client.

Simply go to this page: http:/crm/sdk/list.aspx

Note: Insert the name of your CRM server in the link.

Clicking on the Definition icon will show the Attributes and Relationships for a particular entity in a worksheet-style view.

I've found this very handy for not only documentation purposes, but to get a quick glance at the definition of a particular Entity.