I've been running a Dungeons & Dragons campaign online for over three months now, mostly using Discord since a few of my players enjoy its dice-rolling bot Avrae. And though we often imagine action scenes using theater of the mind, a phrase I can't say out loud without putting on a voice because it sounds way too overblown for "just describing where the gnolls are", we do sometimes bust out a battlemap and slide tokens around on a grid.
While there are plenty of alternatives, and I've looked into Roll20 and the rest, I am above all lazy. I'd rather spend the limited time I have to prep each week crafting exciting scenes and workshopping the outrageous German accent I'm planning to give Mortzengersturm the Manticore. That's why I've been using Owlbear Rodeo instead.
It's a free virtual battlemap that runs in a browser, and doesn't require a download or a log-in from you or your players (although you can set a password for your room). All I have to do is drop a link in the Discord channel and my players can open their browser and see a gnome skyship or the Circle of Thunder. It comes with default maps you can draw on and a set of generic fantasy tokens, but you can also upload both. Only the host can see the tokens in their tray, but once they're placed on the map the players can move them around as they try to manoeuvre into the perfect position to cast burning hands from.
There's a dice roller as well, a box that slides in from the left and lets you bounce polyhedrons around. The results show up as a number near your name, because I suspect other players being able to watch the physics-enabled dice actually clatter about would be too much for something as lightweight as Owlbear Rodeo.
It also features fog of war, a countdown timer, measuring tools for when your map doesn't have a grid, a glowing pointer, and a currently experimental option for sharing the host's audio if you want to DJ while you DM. I've tried a couple of those tools as I need them, but the main reason I use Owlbear Rodeo is that it's powerfully simple. I'm not looking for a full suite of videoconferencing and character-sheet hosting systems, or a rules wiki or pre-made spell templates. In fact, the absence of too much D&D-specific stuff means I'm more likely to keep using it next time we play a different RPG.
Owlbear Rodeo was created by two roleplayers who moved their campaign online during the COVID-19 pandemic. They're still updating it, recently adding enhancements for mobile and more accessibility options, but in its current state it already does everything I need it to. Plus, it's got me tempted to put some rideable owlbears in a future adventure.