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 layer and overlay plugins to choose from.

Plugin Description
Leaflet.markercluster A beautiful, sophisticated, high performance marker clustering solution with smooth animations and lots of great features. Recommended!
MaskCanvas A canvas layer that can be used to visualize coverage.
leaflet-usermarker A plugin for plotting a marker representing a user – or multiple users – on a map,
with support for drawing an accuraccy circle. Can be seen in action on
Longitude.me.
Leaflet.EdgeMarker Indicate the existence of Features outside of the current view.
WebGL Heatmap A high performance Javascript heatmap plugin using WebGL.
Leaflet.Geodesic Draw geodesic (poly)lines. A geodesic line is the shortest path between two given positions on the earth surface. and You can also calculate the exact distance between two given points on the map.
Leaflet.LineExtremities Show symbols at the extremities of polylines, using SVG markers.
Leaflet.MultiTileLayer Compose a TileLayer from several tile sources. Each source is active only on a defined set of zoomlevels.
Leaflet.AnimatedMarker Animate a marker along a polyline.
Leaflet.ellipse Place ellipses on map by specifying center point, semi-major axis,
semi-minor axis, and tilt degrees from west.
Leaflet.plotter It allows you to create routes using a leaflet powered map. You can click on the mid-points to create a new, draggable point.
Leaflet.label Adds text labels to map markers and vector layers.
RaphaelLayer Allows you to use Raphael as a layer on a Leaflet map for advanced animations and visualizations.
Overlapping Marker Spiderfier Deals with overlapping markers in a Google Earth-inspired way by gracefully springing them apart on click.
TileLayer.BoundaryCanvas Draw tile layers with arbitrary polygonal boundary. HTML5 Canvas is used for rendering.
HeatCanvas A simple heatmap api based on HTML5 canvas.
heatmap.js A JavaScript Library for HTML5 canvas based heatmaps. Its Leaflet layer implementation supports large datasets because it is tile based and uses a quadtree index to store the data.
Leaflet divHeatmap Lightweight and versatile heatmap layer based on CSS3 and divIcons
Leaflet-semicircle Adds functionality to L.Circle to draw semicircles.
Leaflet.FunctionalTileLayer Allows you to define tile layer URLs using a function, with support for jQuery deferreds.
Leaflet.geoCSV Leaflet plugin for loading a CSV file as geoJSON layer.
Leaflet.PolylineDecorator Allows you to draw patterns (like dashes, arrows or evenly spaced Markers) along Polylines or coordinate paths.
Leaflet.Sprite Use sprite based icons in your markers.
Leaflet.BounceMarker Make a marker bounce when you add it to a map.
Leaflet.TextPath Allows you to draw text along Polylines.
Leaflet.Awesome-Markers Colorful, iconic & retina-proof markers based on the Font Awesome icons/Twitter Bootstrap icons
Leaflet Data Visualization Framework New markers, layers, and utility classes for easy thematic mapping and data visualization.
TileLayer.Grayscale A regular TileLayer with grayscale makeover.
TileLayer.GeoJSON A TileLayer for GeoJSON tiles.
TileLayer.Zoomify A TileLayer for Zoomify images.
TileLayer.DeepZoom A TileLayer for DeepZoom images.
Leaflet.Graticule It draws a grid of latitude and longitude lines.
Leaflet.Shapefile Put a shapefile onto your map as a layer.
Leaflet.FileGDB Put an ESRI File GeoDatabase onto your map as a layer.
Leaflet.Terminator Overlay day and night regions on a map.
Leaflet.FeatureSelect Use a configurable centerpoint marker to select any geometry type from a GeoJSON layer.
Leaflet.MakiMarkers Create markers using Maki Icons from MapBox.
Leaflet.Editable.Polyline It provides editable polylines: move existing points, add new points and split polylines.
leaflet.TileLayer.WMTS Add WMTS (IGN) layering for leaflet.
leaflet-omnivore Loads & converts CSV, KML, GPX, TopoJSON, WKT formats for Leaflet.
Leaflet.TileLayer.IIP Add support for IIPImage layers in Leaflet.
Leaflet.ImageTransform Add support of image overlays with arbitrary perspective transformation.
PruneCluster A fast and realtime marker clustering library.
Leaflet.Indoor Create indoor maps such as rooms and corridors.