Role Capabilities: Add New Ones?

User Role Editor plugin allows you to create custom roles easily, and if you wish, change any standard WordPress user role (except administrator) as well. This plugin is very well maintained.

But just in case you haven’t, I would like to suggest that you first consider reading about the various pre-defined Roles in WordPress and their capabilities — WordPress Codex: Roles and Capabilities.

EDIT: Role Scoper allows you to assign restrictions and roles to specific pages, posts or categories.