Manually installing Plug-ins
To install, just copy the plug-in to /System/Library/CoreServices/Finder.app/Contents/PlugIns on your Apple TV.
After it's in place on the Apple TV, you need to restart Finder (either with killall Finder if you have killall installed, otherwise use ps awx|grep Finder to get the PID and then kill PID.)
Automatically installing plug-ins from the directory
http://plugins.awkwardtv.org/ (Submit yours here)
ATV_Loader is a plug-in that accesses this directory and installs/updates plug-ins online.
This is probably the easiest method of getting/updating plug-ins. So you need to install one plug-in manually only, namely ATV Loader, which will in turn allow you to install all the other plug-ins from a GUI.
Ideas for Plug-ins
(Please comment if you are working on one of these)
Without fully knowing what kind of capabilities are possible, here are some suggested plugins. Someone with some more development knowledge can perhaps update this section letting us know what capabilities we have here (can we run any objective-c code? Can we display anything, or are we limited to the list view?) Can we enable dashboard over the ATVs default finder app? that would take care of many of these small plugins.
FYI #1: When adding plug-in ideas please keep in mind that a plug-in (aka appliance) is for the most part it's own UNIQUE entity. What does that mean? In short once a specific .appliance has been selected/chosen from the AppleTV main menu, that process/program/plugin/appliance is in charge. In other-words only ONE SINGLE 'appliance' can be in operation at any one time. It is however QUITE possible for a appliance author to write an appliance that will in turn launch any number of other OS X (command line **AND/OR** GUI) applications.
FYI #2: Plugins requests that deal with things that 'pop stuff up' over the top of something that is already playing movie, music, photo slideshow, etc... A plug-in (appliance) can not modify or add functionality to an existing plug-in. For example an oft requested plug-in is a 'CALLER-ID' plug-in. This simply can NOT EXIST (not in the form of a plug-in anyway) - remember if someone did write a growl (or caller-id) plugin then once you 'enter it' it would also have to have the ability to play movies, music, photo slideshows etc because you couldn't use the Apple supplied ones (not without exiting out from the CALLER-ID plug-in first).
FYI #3: Doesn't the AppleTV have a screen saver that pops up over the top of ANY plug-in no matter what plug-in is running? Yes as a matter of fact it does BUT the screen-saver is NOT a appliance/plug-in its some other form of addon. It's not know (to this author) if the way screen-savers work would be a way to implement things like CALLER-ID type functionality.