From the manual: (Plug-in API)

When launched, TextMate loads plug-ins located in ~/‍Library/‍Application Support/‍TextMate/‍PlugIns. The plug-in should be a normal Objective-C bundle with a principal class which will receive an initWithPlugInController: when instantiated.

List of plugins for TextMate:

  • BlogMate - BlogMate is a free, simple, plug-in for the popular TextMate editor that allows creating Inspirational Motivational Quotes and editing blog posts for some types of blogs from a floating palette within TextMate.
  • TmCodeBrowser - TmCodeBrowser is a TextMate plugin designed to help navigating source files. It will parse any language known to the underlying Exuberant Ctags program (enhanced by a small script to also support Objective-C). It will present classes, subroutines etc. in a TreeView. Clicking on any entry will jump to the corresponding location in the source code.
  • WebMate - WebMate(Dead Link) turns TextMate's Web Preview window into a WYSIWYG editor, using the built-in editing capabilities of WebKit. After installing, go into the "Show options" drawer of the Web Preview window and check the "Allow live editing" checkbox. Changes that you make in the preview window will automatically update your source document. For big changes, this will likely make your source harder to read, but it really does an amazing job for generated HTML. You can always Undo the changes in the source window.
  • TabMate - TabMate lets TextMate make it possible to have per-file editing options (indenting settings, language etc), that helps keep code consistently formatted, even when hundreds of people might be hacking it with their text editors of choice. Now also provided as Universal Binary.
  • XMLMate - XMLMate checks XML and XHTML documents for Well-Formedness and Validity while editing them in TextMate with support for DTD, W3C XML Schema, RELAX NG, Schematron and XInclude.
  • XSLMate - XSLMate Turn TextMate into an XSLT/XQuery editor/debugger.
  • MissingDrawer - MissingDrawer replaces the project drawer with a fixed project navigation pane, on the left (screenshot).

    See the newer version on GitHub.
  • ProjectPlus - ProjectPlus combines and improves MissingDrawer with SVNMate (though I don't know how it compares to the "newer version" of MissingDrawer metioned above).
  • ViMate - ViMate (dead link) brings VIM modal-editing and shortcuts to Textmate.
  • ReMate - ReMate disables the automatic project tree refresh that occurs after switching between running applications. Very useful when editing files that live on a network mount.
  • nice_find - nice_find a plugin for faster, asynchronous Find In Project
  • TMGoToLastEdit - TMGoToLastEdit a plugin to place the caret at the last edit position
  • Textmate-Minimap - Textmate-Minimap shows a minimap of the current document in a new drawer. Can be used for visual text navigation or quickly recognizing documents by their 'shape'.
  • Textmate-Fullscreen-Plugin - Textmate-Fullscreen-Plugin enables you to edit files in fullscreen-mode.
  • TextMateVim - TextMateVim - the beauty of TextMate meets the power of Vim.