Helpful Non-Core Content?

[Urpriest]

I've DMed a lot of 3.5 and a lot of 4e, but only a little 5e. It's something I'd like to get better at, as 5e seems great for teaching new players. I haven't been keeping up with new 5e content at all though, which brings me to my question:

Is there any 5e content past the core books that's really helpful, and makes the game run a lot smoother? I remember in 4e WotC changed how they designed monsters partway through, so later monsters led to much better combats than those in the original Monster Manual. Is there anything in 5e like that? (I remember being particularly frustrated with the swingyness of 5e's monsters. :smalltongue:)

[P. G. Macer]

I don’t think there’s anything quite like that in 5e, though Xanathar’s Guide to Everything provides alternate rules for a few of the clunkier things in the DMG, such as crafting and downtime. Tasha’s Cauldron of Everything is also very popular among the player base, though online it is also extremely controversial.

[Grod_The_Giant]

The only really transformative thing I can think of are the cantrips Booming Blade and Green Flame Blade, which appear in both Sword Coast Adventurer's Guide and Tasha's Cauldron of Everything. Unlike pretty much every other spell in the game, they have you make a melee weapon attack as part of the spellcasting action. Thus, they deal normal weapon damage (which is going to be higher than standard cantrip damage until at least level 11), have a rider effect, and get extra damage dice on both the rider effect and the base attack at 5th, 11th, and 17th level.

The upshot is that they providing a scaling melee attack that's still worth using at higher levels, and which don't require investment in a secondary ability score if you don't feel like it. Great for Rogues, Clerics, and any sort of generalist spellcaster. Think of them as the equivalent of having medium BAB in 3.5-- they're not exactly competitive unless you've got something else going on, but you won't feel bad using it. And they're amazing for anyone who can manage to mix melee and spellcasting, such as the Valor Bard, Eldritch Knight, and Sorcerer.