Note that this list is by no means complete. If you don’t find your own mentioned here, feel free to comment below. I’ve also tried to roughly group them so you can find them later more easily.
Authentication & Security
li3_access: Access control library by tmaiaroto.
li3_ids: Intrusion detection integration (phpids) by weluse.
li3_oauth: OAuth library by gwoo.
APIs and Externals
li3_delayed_job: Ruby’s Delayed::Job port to Lithium by cgarvis.
li3_facebook: Facebook API wrapper by tmaiaroto.
li3_flickr: Flickr API wrapper by jacopKane.
li3_swiftmailer: Interface to the Swiftmailer library by greut.
View Layer & Helpers
li3_analytics: Integrate Google Analytics easily by greut.
li3_design: Useful things for designers like placeholder texts by UnionOfRAD.
li3_flash_message: Flash message generation by michaelhue.
li3_gravatar: Gravatar integration by michaelhue.
li3_htmlpurifier: Wrapper around the HTML filtering library by harikt.
li3_injector: DOM markup injection by indiefan.
li3_less: Support for LESS CSS by glaszig.
li3_markdown: Wrapper for PHP Markdown by sandelius.
li3_paginate: Pagination plugin by primetheus.
li3_pdf: PDF generation plugin by greut.
li3_sitemap: Sitemap generator by masom.
li3_twig: Twig templating support by henrikbjorn.
Data Layer
li3_activerecord: PHP ActiveRecord wrapper by greut.
li3_behaviors: Model behavior support by nateabele.
li3_dateable: Adds timestamp to records/documents by weluse.
li3_doctrine: Doctrine2 integration by Richard McIntyre.
li3_geo: Geospatial querying for multiple databases by nateabele.
li3_simplesearch: SQLite3 based searching by raisinbread.
li3_sqlsrv: SQL Server datasource by jperras.
li3_tree: Tree model behavior by vogan.
Testing & Documentation
li3_docs: Official documentation plugin by UnionOfRAD.
li3_fixtures: Add Fixtures to your tests by daschl.
li3_junit: JUnit integration by Michael Harris.
Misc
li3_cldr: Query the Unicode CLDR by UnionOfRAD.
li3_config_loader: Load configs on the fly during bootstrap by Howard3.
li3_debug: Debugging Library by Howard3.
li3_rest: adds REST support to your application by daschl.
I have to say that I’m very excited to see what the community is currently building and how Lithium as a whole evolves. Thanks to everyone who is participating and keep up the awesome work!