There are many modules that use or complement Flag.
- Views: Create lists of flagged content.
- Flag Weights: Add weights to Flags to make items orderable.
- Flag Friend: A pre-built flag for marking other users as friends with Flag.
- Flag Abuse: A pre-built flag for marking offensive content, plus administrative features.
- Flag Terms: Adds the ability to flag taxonomy terms (Drupal 6; this is part of Flag on Drupal 7).
- Flag files: Adds the ability to flag files.
- Flag Service: Allows Flag to be accessed via a web services interface through the Services module.
- DLike: Provides a 'Like' button using a flag.
- Conditional Flags module provides an extra API to Flag module, for custom conditions between flags.
The above is a partial list. Feel free to add to it, and create documentation for them as child pages.