It would be fantastic if there was a centralized location for all the different Litihium plugins. It could simply be a page on the lithium website with titles, description, a link to the plugin home page, and/or a link to a git repo for the plugin.
Or, possibly, you could create another Github organization, like "lithium_plugins" that hosts the master repo for each plugin. But that sounds as though it may be a little less practical, since (I think?) you would run into permissions issues for all the different authors for their plugins.
li3_lab (https://github.com/UnionOfRAD/li3_lab) aim is exactly that !
It's live, and needs some love: http://lab.lithify.me/
This search might be useful: https://github.com/search?langOverride=&language=PHP&q=Lithium&repo=&start_value=4&type=Repositories&x=28&y=24
ok i know li3_lab exist but i dont have time to read code and think how its work, this proces, write and public plugins, extensions should be write in manual
The lab is phasing out, instead we recommend to search github using the li3_ prefix or look at the UOR account for "official" plugins. Alternatively there's also lithium101.com.