Leaflet is designed to be as lightweight as possible and focuses on a core set of features. You can extend its functionality through third-party plugins. Here are several Leaflet plugins to choose from.

Plugin Description
Proj4Leaflet Proj4js integration plugin, allowing you to use all kinds of weird projections in Leaflet.
Plugins by Pavel Shramov A set of plugins for: GPX, KML layers; Bing tile layer; Google and Yandex layers (implemented with their APIs), permalink and distance measurement controls.
cartodb-leaflet Official CartoDB plugin for Leaflet.
Leaflet Vector Layers Allows to easily create vector layers from a number of geo web services, such as ArcGIS Server, Arc2Earth, GeoIQ, CartoDB and GIS Cloud.
leaflet-tilejson Adds support for the TileJSON specification to Leaflet.
leaflet-providers Contains configurations for various free tile providers — OSM, OpenCycleMap, MapQuest, Stamen, Esri, etc.
azgs-leaflet A set of small plugins for Leaflet, including WFS-GeoJSON layer with filtering, a hover control for GeoJSON, and an Esri tile layer.
Leaflet.encoded Use encoded polylines in Leaflet.
Leaflet.Pouch Use PouchDB to sync CouchDB data to local storage (indexedDB), to just add couchDB data or as just a less confusing implementation of indexedDB.
Leaflet Ajax Add GeoJSON data via ajax or jsonp.
Leaflet GPX GPX layer, targeted at sporting activities by providing access to information such as distance, moving time, pace, elevation, heart rate, etc.
Wicket A modest library for translating between Well-Known Text (WKT) and Leaflet geometry objects (e.g. between L.marker() instances and “POINT()” strings).
Leaflet.dbpediaLayer A layer with Points of interest from Wikipedia – loaded via ajax from DBpedia’s SPARQL endpoint.
Leaflet-2gis Adds support for 2GIS tile layer
Leaflet.KoreanTmsProviders Contains configurations for various (South) Korean tile providers — Daum, Naver, VWorld, etc.
Leaflet.ChineseTmsProviders Contains configurations for various Chinese tile providers — TianDiTu, MapABC, GaoDe, etc.
Esri Leaflet A set of tools for using ArcGIS services with Leaflet. Support for map services, feature layers, ArcGIS Online tiles and more.
Leaflet.geojsonCSS Geojson CSS implementation for Leaflet.