We have seen a lot of different functionalities that Ogre 3D offers, but Ogre 3D also makes it quite easy to extend it with new functions. That's the reason there are a lot of different libraries that can be used to add some new functions to Ogre 3D. We will discuss some of these libraries to get a feeling for what add-ons are out there. A complete list can be found in the wiki at http://www.Ogre3D.org/tikiwiki/OGRE+Libraries.
Speedtree is a commercial solution used to render a lot of good-looking trees and grass. It is widely used by several commercial games and the founder of Ogre 3D Sinbad offers a binding for Ogre 3D. Speedtree and the binding for Ogre 3D must be bought and aren't freely available. More information can be found at http://www.ogre3d.org/tikiwiki/OgreSpeedtree.
Hydrax is an add-on that adds the capability of rendering pretty water scenes to Ogre 3D. With this add-on, water can be added to a scene and a lot of different settings are available, such as setting the depth of the water, adding foam effects, underwater light rays, and so on. The add-on can be found at http://www.ogre3d.org/tikiwiki/Hydrax.
Caelum is another add-on, which introduces sky rendering with day and night cycles to Ogre 3D. It renders the sun and moon correctly using a date and time. It also renders weather effects like snow or rain and a complex cloud simulation to make the sky look as real as possible. The wiki site for this add-on is http://www.ogre3d.org/tikiwiki/Caelum.
Another commercial add-on is Particle Universe. Particle Universe adds a new particle system to Ogre 3D, which allows many more different effects than the normal Ogre 3D particle system allows. Also, it comes with a Particle Editor, allowing artists to create particles in a separate application and the programmer can load the created particle script later. This plugin can be found at http://www.ogre3d.org/tikiwiki/Particle+Universe+plugin.
There are a lot of different GUI libraries available for Ogre 3D, each of which has its reason to exist, but there isn't one GUI library everyone should use. The best thing is to try out some of them and then decide for ourselves which library suits our needs best.
CEGUI is probably the first GUI library that has been integrated into Ogre 3D. It offers all functions expected from a GUI system and a lot more. There is a GUI editor to create your GUI outside of code and a lot of different skins to customize your GUI. More information can be found at http://www.cegui.org.uk/wiki/index.php/Main_Page.
BetaGUI is an extremely small library, which comes in one header and one cpp file. The only dependency is Ogre 3D and it offers basic functionality like creating windows, buttons, text fields, and static text. It is not a complete GUI, but it offers basic functionality without any dependencies, so it can be used when a simple and quick solution is needed. More can be found at http://www.ogre3d.org/tikiwiki/BetaGUI.
QuickGUI is a more complex and powerful solution than BetaGUI. Though QuickGui offered a lot more widgets, it also made the integration process a bit more difficult. QuickGUI is a full-blown GUI solution that can be used for all kinds of different projects and is updated regularly. The wiki site can be found at http://www.ogre3d.org/tikiwiki/QuickGUI.
Berkelium isn't a GUI library as such, as it doesn't have any widgets or anything similar. Instead, it enables Ogre 3D to render websites using the Google Chromium library. With the help of this library, it is possible to build an in-game web browser. The website can be found at http://www.ogre3d.org/tikiwiki/Berkelium.