The Plugins section isn't really that fancy; there's no proper API to interface with HTML³ from outside programs yet, so the Plugins list is simply a list of software and resources that you might want to get at quickly when you're designing web pages.
For instance, you might want to keep the Argo counter creator handy if you subscribe to ArgoNet, and selecting it from the Plugins list will launch that program; as there's no iconbar icon the main window of this program will open, and when you're done with it you close the window and the program quits, it's almost as if it's part of HTML³ except that it's not in memory all the time and so isn't taking up valuable resources until it's needed. As you'll probably only ever use it when you're designing web pages so it makes sense to keep the two programs together.
You can also use it to load other programs you might want to use at the same time, such as InterGIF for GIF creation, an image map creator, a link checker, DES encryption program for generating password protection for our web site, a directory with images in and so on - there's nothing stopping you keeping these in other parts of your hard drive, but experience with ArgoNet's Voyager shows that there's a lot more Voyager add-ons to load third-party programs from the Voyager bar than there are "proper" add-ons that do something unique to Voyager.
There's one special feature that you might want to use though; by having a text file called _Plug-in inside your applications, you can give the application a more verbose description (up to 25 characters, with spaces and other "illegal" characters that you can't get in a filename). Otherwise the application's filename, minus the leading "!", will be used - it's up to you if you want Imp2HTML or Impression DDF to HTML as the description for that program.
I do intend to put together some kind of API to allow other programs to use HTML³'s data to be used - for instance, a Style Sheet plugin that knows which page you're working on and can supply HTML³ with data to pump through to the text editor without duplicating lots of code that's already in the original program - but it's lower on the priority scale than finishing the main program and getting it properly documented.
HTML³: © Richard Goodwin 1997-2002