Ethereum's official Gitter is one of the best places to find fellow Ethereum developers to chat about topics such as Solidity, Web3.js or EIPs (Ethereum Improvement Proposals).
Protip: If you're already a Matrix user you can use their Gitter Bridge to join Gitter rooms via your Matrix client.
The /ethdev subreddit is one of the most active forums where more than 40k developers discuss everything related to Ethereum development.
Browsing through the subreddit not only do you find the latest developer-related information and tutorials, you can also ask questions when you get stuck or showcase the project you've been working on.
Protip 1: Use the "Filter by flairs" feature on the right hand side to find the posts you're interested in.
Protip 2: Take note of the "Stickied posts" where you'll find a "Who's hiring, and who's for hire" thread.
While the OpenZeppelin Forum is one of the best places to discuss subjects concerned with Smart Contract security, it's also a great community to join if you're interested in Ethereum and dApp development in general.
You might want to start by browsing through the "General" subforum which covers different topics such as "Guides and Tutorials", "Project", "Showcase", etc.
Protip: Search for the "coding-journey" tag to read how fellow dApp developers navigated the Blockchain space and got started with Smart Contract development.
If you're really deep into bleeding edge Blockchain and Crypto technologies you'll enjoy the official Ethereum Research Forum where researchers of the Ethereum Foundation and community members discuss the ways in which Ethereum can and should evolve in the future.
While it can be overwhelming to navigate the sheer amount of information on the forum you can start by narrowing your search down through filters and categories. Both of which you can find at the top.
The Ethereum Magicians forum is a place for Ethereum users and developers to gather and connect. Discussions are mainly focused on technical difficulties of the Ethereum ecosystem as well as EIPs.
Anyone can join and contribute by attending discussions, providing feedback, starting or joining a working group or sharing code and prototypical ideas.
Joining the ETHGlobal Discord server you'll get access to a welcoming community of developers working on and with Ethereum technologies. Use this Discord to discuss the current development of the Ethereum Blockchain, ask questions regarding Smart Contract and dApp development or find your next developer job in the space.
The solidity-dev Discord server is a dedicated place for developers working with the Solidity Smart Contract programming language. Join this server to ask for help when you get stuck or discuss tips and tricks uncovered while working with Solidity and its tools.