The Trapcode Suite by Red Giant is one of the most notable plugins available for Adobe After Effects. This suite contains many plugins which range from being useful to crucial for After Effects professionals.
The two plugins that I use the most are Particular and Form.
Particular is perhaps the most famed plugin in the After Effects community. It is a powerful particle engine, which is extremely customization and robust features such as turbulence, physics, time maps, and 3D integration. My favorite feature of Particular is allowing particles to emit from lights, which makes it easy to make repeatable particle effects. This plugin is far easier to use and much more customization than After Effects' built-in CC Particle World.
Form is another particle engine, but works differently. All particles exist from the beginning to the end of the layer; that is to say, there is no birthing or dying of particles. One of the most notable features of Form is that it be driven by sound. This is useful for lining up action with a sound effect, or to quickly drive organic animation. Form can also be applied to 3D .obj models, which can create beautiful HUD-like images or create interesting shapes.
The many other plugins in this suite allow you to create 3D imagery, create stylized compositing, or utilize useful tools.
I often use Shine for creating light rays or adding stylized hot-looking backgrounds. I don't often use the others, but each is useful in their own right.
All together, the Trapcode suite is a powerful set of plugins that is critical for any After Effects artist, and can have applications in visual effects and motion graphics.
While the 3D integration is one of the most useful aspects of the suite, it has its breaking points. The plugins will adhere to the 3D camera, but it can be tricky to integrate them in proper 3D space if another 3D layer is supposed to pass through particles. Particular allows for one "obscure" layer which can be placed between particles, but Form and others do not support this option.
The plugins that create 3D mesh such as Mir and Tao can be difficult to figure out, and they don't create the best looking 3D shapes. The lighting and shading can look very flat and computer-generate. If you want really good looking 3D integration, you should pick a dedicated 3D program.
There was an instance where I wanted to use a plugin called Plexus to create a 3D hologram-like effect. I found the plugin to be too complicated to use, and beyond that, much too taxing on my GPU. I nixed the idea and switched to the more familiar Trapcode Form. Form was much more easily able to handle a mesh with tens of thousands of vertices, and helped me achieve nearly the same effect.