Leaflet is a wonderfully lightweight mapping platform. However the functionality of the platform can be extended by using any number of third-party plugins. The documentation for Leaflet includes a great list of some of the available third-party plug-ins. Here are a few more that you might find useful.
Leaflet.draw is a vector drawing and editing plugin for Leaflet. The library adds a drawing toolbar to a Leaflet map. The toolbar allows users to then add markers and draw polylines, polygons, circles and rectangles on the Leaflet map. Leaflet.draw is included in the plug-in list in the Leaflet documentation but I've also included it here because of the Leaflet.Toolbar plug-in.
Leaflet.Toolbar is a plug-in which allows you to build and add a flexible and extensible toolbar to a Leaflet map. Leaflet.Toolbar extends the Leaflet.draw plugin to enable custom user interactions. You can view an example of Leaflet.Toolbar on this map.
Leaflet List Markers is a handy library for providing a dynamic menu for all the markers displayed in the current map view. When users adjust the map view the menu automatically updates to list only the markers in the new map view.
Leaflet.TransitionedIcon is a plug-in that allows markers to be eased in and out of view on a map. The library allows you to fade markers on and off a Leaflet map. You can see the marker transitions in use on this demo map.