A legend widget for doing range filtering of numeric attributes of a QGIS layer. Motivated by needing to explore a large dataset, I wrote this question: https://gis.stackexchange.com/questions/328818/using-qgis-slider-for-filtering-based-on-attributes/329109#329109
You can get the release version of this plugin from the QGIS Plugin Repository in QGIS or here:
https://plugins.qgis.org/plugins/legend_data_filter/
If you want the absolute latest version, you can install this plugin by copying the folder into your QGIS plugins folder, and then adding the "Data Range Filter Legend Widget" plugin. The widget is then available in the layer legend widgets.
You can see an early version of this widget in action here: https://www.youtube.com/watch?v=0FapVK6dpC8&feature=youtu.be