The recent update for Pokémon GO is one of its biggest yet, and totally revises the way gyms work in the game. As well as making alterations to how monsters are assigned to a gym, these locations are now home to Raid Battles, which occur periodically throughout the day.
Each Raid Battle only lasts for an hour so serious players need to keep abreast of when and where they're taking place - and that's where the updated GymHuntr site comes in.
This scans any location in the game and tells you of upcoming raids, ongoing raids and basic information about the status of each gym, such as which team is currently holding it and what level of prestige it has.
Niantic has previously taken quite a dim view of apps like GymHuntr, so there's no telling if the company will attempt to shut it down - especially now that the site's new functionality makes it even more useful and the increased traffic could have a negative effect on Pokémon GO's servers. Time will no doubt tell, but right now this is a very useful resource for planning your future raids.