Here are various links for plug-ins for After Effects:
You would be amazed at what you can do with the effects that come bundled with AE. There are only a few free ones out there. Red Giant has a couple of handy utility plug-ins that are free. Most 'free' plug-ins are worth exactly what you paid for them while some offer an easier way to do some common tasks.
What exactly are you trying to do? Experimenting with eye-candy is not a very productive way to learn AE. Breaking down an interesting composite or sequence then trying to duplicate it is a very effective way to learn.
As Rick already said, it all depends on what you actually want to do. Even commercial plug-ins will require a technical and creative understanding of how they work and what they can do and aside from providing presets to illustrate their features or facilitate everyday tasks, few of them will have any magic of their own...
I agree with the others here. Most of what you want to do can be done with the things that come with AE. They just require some work. If you're new to AE, I strongly suggest that you start here. It may seem boring at first, but it is absolutely essential to actually being able to work with AE.
Put the .aex file into the Plugins folder inside the After Effects application folder. You can create your own folder within the Plugin folder if you want to, for tidyness. Restart After effects to initialise the plugin.
A .dmg file is a compressed disk image, usually for Mac users only. It must be mounted (opened) to get the files inside.
Mac plugins won't work in Windows and vice versa.
I did add the .aex file into my plugins folder-Effects Folder, and I restarted AE, and could not see it in my Effect panel. I also added this plugin name mi_atlas, and I dont see it.
I also created a folder inside the Plugins called it AEFlame, and copied the Plugin there, still dose not show.
Here Is the screen-shot from my plugin folder.
Thank you very much.