I'd like to use QGIS to explore and download Planet Labs imagery. Is there any way to do that?
Yes there is. Luiz Motta made a plugin for QGIS to talk to the Planet Labs API, called Catalog Planet Labs. It can be installed directly from QGIS, though only version 2.8.0 and higher. Just open the 'plugins' menu, adjust the settings to 'show also experimental plugins', and then search for 'Planet Labs'. The code is all up on GitHub, and contributions are welcome.