There are so many plugins that you can purchase for After Effects that allow you to do incredible things. However, with the economy hurting and everybody looking for a little help, Topher has created an AE Plugin Stimulus Package to give your projects a free little boost!
Scott Frizzle's Quaker
Scott is a great broadcast designer working in the industry for over 15 years for some of the top networks on the air today such as Cartoon Network, TNT, and Discovery Channel. Scott developed Quaker as a plugin to simulate earthquake effects. He also has a ton of project files and presets for Trapcode Particular as well from the days when Trapcode People was around.
Normality 3 is a powerful plug-in that allows CG Artists to light 3D objects and scenes directly within the familiar Adobe After Effects environment through the use of normal passes and advanced relighting techniques. They offer a great FAQ and some great tutorials on how to use the plugin.
Red Giant Unmult and eLin
Red Giant software has a gigantic arsenal of plugins, but there are a few that actually are free to download that can help out some everyday tasks. First they have Knoll Unmult, which is a free little plugin that comes along with Knoll Light Factory to knock out the black on solids with lens flares on them. Red Giant has made this free to download since it is such a simple, yet useful utility. They also have eLin, which is used to help you when compositing 3d elements with live action by simulating real world motion blur, lighting, and can also work with OpenEXR.
Buena's AEFlame and Swatch Buckler
Buena makes that old AEFlame plugin which kind of doesn't have that many great uses since all it really does is generate fractal looking flame-ish looks, but Buena has a great little plugin called Swatch Buckler that actually gives you a floating palette that you can grab colors from... a lot easier than trying to remember color codes or eyedroppering every fifteen seconds.
Fnord has a bunch of really nice little plugins that aren't really effects by any means, but extensions that you can use to get more out of your formats within After Effects. First is EXRDisplay, which gives you more control over the way your EXR images can display by messing with different gamma and exposure options not natively available. Of course you have to have OpenEXR installed on your system to read the images. The J2K plugin just lets you actually read and import JPEG2000 files, the successor to JPEG. SuperPNG is also another extension that gives you more advanced options over reading and writing PNG files, and SuperTIFF does the same thing, but for TIFF formats.
GridIron Software X-Factor
GridIron Software, the creators of Nucleo and Flow have a free plugin that can speed up your rendering and preview times by 100%... cut your time in half! That is if you have two additional CPU's and some extra RAM, but there isn't a reason why you shouldn't have this plugin if you meet the criteria.
This is quite handy if you like to preview what you are working on, on a separate full size monitor. This plugin lets you take DV or D1 footage inside your composition and preview that exactly as it would be seen on television on a studio monitor for more precise editing and tweaking. This is quite helpful if you are doing intense color correction or pinpoint precise motion tracking or roto jobs.
The Omino Suite is a collection of basic tools, all of which serve a very unique purpose, and can save you a few minutes here and there. Mino Colorizer colorizes the footage, Diffusion is a color matcher, Feedback simulates bad tv signals, Squares reconstructs your image into a ton of little squares. Sphere Map is designed to help with distant star fields and the like, and Halftone recreates a retro halftone effect.
Obvious FX Copy Image
Lastly, this is a nice little alternative to using the alt+printscreen buttons to get a screenshot of what I am working for to send clients. This plugin copies just the image in your composition window to your clipboard, and then you can paste it into an email, or whatever you need instead of bringing a full screen cap into Photoshop to cut it up, save and all those other annoying steps.