Modules for URLs, aliases, paths, and links

Pathauto (http://drupal.org/project/pathauto)

Requires Path, Token.
Provides a mechanism for modules to automatically generate URL aliases for the content they manage. Human-friendly URLs (e.g.,"about" rather than "node/72") are important for accessibility, usability, and SEO.

Path redirect (http://drupal.org/project/path_redirect)

Redirects users from one URL to another. When used with Pathauto, provides a new "Update Action" for when URL aliases change (e.g., when a node's title is changed). This is the recommended update action and is considered the best practice for SEO and usability.

Elements (http://drupal.org/project/elements)

Provides a library of Form API elements for use by other modules. For example, it improves Path redirect's administration screen with checkboxes, a select-all checkbox, and bulk operations.

Global Redirect (http://drupal.org/project/globalredirect)

Searches for an alias of the current URL and 301 redirects if found. Stops duplicate content arising when Path module is enabled.

Link checker (http://drupal.org/project/linkchecker)

Periodically checks for broken links in node types, blocks and cck fields and reports the results.

Pathologic (http://drupal.org/project/pathologic)

Requires Filter.
Helps avoid broken links and incorrect paths in general text fields (e.g., Body, etc.). These tend to arise when full URLs ("http://sample.edu/about") or relative path URLs ("about") are used in general text fields instead of absolute path URLs ("/about") for internal content.

Linkit (http://drupal.org/project/linkit)

Needs Pathologic and Wysiwyg or CKEditor.
"Linkit provides an easy interface for internal linking" in general text fields (e.g., Body, etc.).

Linkit Picker (http://drupal.org/project/linkit_picker)

Requires Linkit, Views.
Extends Linkit with views listing existing internal content for selection (like Node Picker had).