User Role Editor is an useful plugin that allow you to change users and roles capabilities very easiy. You can edit/add/delete WordPress user roles and capabilities from the User Role Settings:
Until the version 1.9, capability changes have affected only the back end permissions. From this version is possible to use the meta capabilities also to override the front end permissions.
In Appearance > Theme Settings > Permissions you can find this option for wiki and posts:
If this option is enabled, the options above about roles permissions won't have any effect for that specific post type. They will be overridden by the WordPress capabilities assigned to the current role/user.
By default the permissions for wiki assigned to the roles follow the standard of WordPress caps for blog posts permissions. Below there is a list of the default capabilities for wiki posts, assigned to Editors, Authors and Contributors (Administrators of course have always all capabilities):
