The Shortcode addon was just released today and it's exactly what you are wanting.
Shortcode aims to allow for more dynamic use of content by authors and
editors, allowing for injection of reusable bits of content or even
whole pieces of functionality into any field in EE. Shortcode provides
two types of codes for use in any content in EE: user-defined Macros
specific to each author and each site, as well as a simple API for
third-party plugins and modules to expose their tags through the
shortcode Rich Text Editor dialog (or manual entry in any type of
field) - complete with automatically rendered options forms.
Two Types of Codes
Macros - User defined, author and site specific bits of reusable content - basically just like Snippets but can they can be embedded
directly in any field type by your content Editors!
Shortcodes - Plugin/Module defined tags that can be injected into the middle of content, such as a ProForm form or other piece of
functionality. This works similarly to the WordPress Shortcode API.
Macros defined by each user for their own use - each author could have the same macros defined with the same names, with different
values that are dynamically replaced based on the assigned author of
Macros defined by Super Admins for the entire site and usable in all content for each site (MSM compatible with separate sets of Macros
for each site).
Macros and Shortcodes can be inserted manually in any fieldtype, or automatically through the included Rich Text Editor tool.
Simple Shortcode API that allows for any Plugin or Module to provide custom Shortcodes complete with automatically generated
Existing plugins and modules can be converted to shortcodes with very little code. Assistance in the forum is available for anyone who
wants to add support to their own add-ons.
Assistance writing glue code for existing plugins or modules is also available - just post a request for support for the plugin in the