Modules

Provides configurable search pages at /search/user and /search/node.

Make sure your permissions are set correctly to avoid leaking information.

Create groups with private content.

There are lots of additional modules with extra features.

The UI can be hard to work with. Modules or custom code are needed to streamline it.

Drupal's main search module.

Use facets with core views.

Multiple facets in a single block.

Configure field validation.

Can check for unique title.

Shows a message, see Unique Field Ajax module for clientside indicator.

Provides a hierarchical select widget for a multi level selection.

Granular content access permissions.

This module tries to do too much and has a lot of issues.

Set content type as private, w/ option to override per node.

User's can grant access to other user's and roles, per node.