Are there any apps which will "watch" constantly and trigger actions when I type a certain string?
There are many ways to accomplish this depending on your needs really. You can even use a combination of things to best fit what ultimately makes you more efficient.

I would recommend the following based upon it's type:

AppleScript


  FastScripts — set hotkeys for any AppleScript by keyboard shortcut or via menubar.


System-Wide Text Macros


  TextExpander - expands custom keyboard shortcuts into text, pictures, commands, etc.


System Events


  LaunchBar — hotkeys file manager, search tool, app launcher, clipboard
  manager, etc.