Reference for custom claims provider

Understanding People Picker and custom claims providers

Custom claims providers for People Picker (SharePoint Server 2010)

How to: Create a Claims Provider

Claims Walkthrough: Writing Claims Providers for SharePoint 2010

People Picker overview (SharePoint Server 2010)

Claims Walkthrough: Creating Claims Providers for Trusted Login Providers for SharePoint 2010

Replacing the out of box Name Resolution in SharePoint 2010 - Part 2