The 2 main ones are:
Agui: http://code.google.com/p/agui/
GWEN: http://code.google.com/p/gwen/

What makes those the "main" ones? What Allegro 5 games have been made with those?

With a little work you could now use GTK. Right now it only works on Linux... but you'd have to find a flexible way of placing the Allegro display widget.

Depends what you mean by features. Offering text boxes isn't very useful for games. General purpose GUIs and game GUIs are different. Mine is completely customizable and built for games, not as a general purpose GUI. And it's been using in half a dozen games or more.

Here's an example of what TGUI2 can do quite easily.

https://www.allegro.cc/files/attachment/607033

Trent, do you have any documentation on TGUI2?

So it doesn't work...

Oh Oh! The little teddy bear got mad and it's going to eat me oh no! help help!

There are a lot of gamers who use Linux. Do you think Valve would be porting Steam if that wasn't the case? The fact still remains, that you're complaining about code I wrote because it doesn't work on your preferred operating system. You're probably too shit of coders to make it work yourself so you complain instead.

Sorry pkrcel, I haven't got around to documenting it yet (in about 10 years since I first created it D:.) I think a good way to figure out how to use it (although documentation would be better) would be to read the source for AshEdit2 here (even though it's not very well done:) http://www.nooskewl.com/content/open-source.

For a few ideas to kick you off, it's designed to be integrated into a game loop easily. The basic flow is something like:

while (true) {
  al_wait_for_event(...);
  tgui::handleEvent(&event);
  if (logic) {
     tgui::update();
  }
  if (draw) {
     /* your drawing goes here, or after tgui::draw if you want */
     tgui::draw();
     al_flip_display();
  }
}

Setup looks something like this:

tgui::init();

ParentWidget = new ParentWidget(...);
ChildWidget = new ChildWidget(...);

tgui::addWidget(ParentWidget);
tgui::setNewWidgetParent(ParentWidget);
tgui::addWidget(ChildWidget);

tgui::setFocus(ChildWidget);

And of course the most important part is creating custom widgets. Basically you want to respond to certain "events" (method calls). Those are listed in tgui2.hpp as the virtual functions. The main ones are probably draw, update, mouseDown, mouseUp, mouseMove, keyDown, keyUp and so on. There are some widgets defined in the library already though only a small number. You can look in tgui2_widgets.cpp/hpp to see more details on how to create a widget. I recommend looking at TGUI_Frame first as that's really simple. It's just a rectangular "window" that you can drag around by the title bar.

GullRaDriel, I'll be implementing a C GUI for my game Real soon Now (tm), so seen that there seems to be a need for it, I'll probably rip it out of my game and make a separate library out of it. Keep watching this place until then...

I have a fully implemented GUI for Allegro 4. It's public too, on sourceforge.

I am in the middle of redesigning my library to become independent of the library it uses, and right now I am finishing the Allegro 5 backend. After that I will begin porting all my widgets to the new library. It is event based, and has some really great convenience functions and objects. It makes input smooth like butter and init code simple. All objects are owned by the system and that is where you create them from, so you never have to manage memory AT ALL! The most you have to do is write one delete statement.

Simple example of how you would display an image and wait for a key with my system, Eagle :