Shamelessly plugging this stuff at the top of the list...
VFA-69 F/A-18C skin
CVW-69 Event Calendar
DCS Simple Radio
Used for communicating with other players on supported servers. It also allows configuration of radios on FC3 aircraft. Free!
That this is free is beyond amazing. Full mission planning and content generation for DCS World. It would be foolish to try and list everything this offers here, so go check it out immediately
Not free, though a demo version is available. LotAtc allows for ATC/GCI overview of airspace on supported multiplayer servers. €25.00 for a single license, bulk discounts available.
Helios allows you to export panels and gauges from DCS onto external displays. It's a great tool for the budding simpit builder, or if you have a few monitors connected and want to have your cockpit displays represented on them. Free.
An interface for allowing custom external hardware and software to talk with DCS via your Export.lua file. Another essential for building your own simpit. Code is maintained on GitHub, is open-source, and has an Arduino library available. Free.
Hoggit on Reddit
A great subreddit made for and by fans of DCS World. Packed with information, links, videos, and news. Also a great Q&A source for newbies and veterans alike.
Maintained by the community, the Hoggit wiki attempts to document some of the more obscure DCS features, such as the API and scripting.
Discord is generally now the de-facto platform for gaming communities, supporting both text and voice channels with game overlay. The Hoggit Discord is one of the key hubs for the DCS community and is highly recommended.
Eagle Dynamics forums
Official forums for Eagle Dynamics / DCS World - support, bugs, feature requests, and news surrounding the game.