The Rep[lacement]Tags module allows you to replace tags (like $MYTAG$ or {DATE}) with user-defined content. It implements two different mechanisms to perform replacements:

1. User-/sidewide tags are managed from the reptag admin pages. Users with appropriate permission can easily add/delete/modify these tags online.

2. Module tags are hardcoded in extension modules. They offer an easy but powerful method for more sophisticated site-specific tags. RepTag comes with several .tags modules for general use cases.

RepTag also provides role-based rights management, online help, support for CCK textfields, integration with other modules and a basic API.

Read more on the project page (and in the documentation).