The official Ethereum Wiki is one of the best places to learn everything about Ethereum as well as related technologies and protocols such as IPFS, Whisper, Swarm, etc.
Browsing through the wiki you'll find plenty of resources on topics like governance, scaling, sharding, Eth1 vs. Eth2, the official roadmap as well as general research related to Blockchains and Smart Contracts.
Protip: The wiki is community-driven which means that you can contribute by joining the discussions and proposing changes.
EthHub is the single source for essential Ethereum information. Browsing through it you can learn what Ethereum is, how the upcoming Ethereum 2.0 will work and what types of applications there are on the Ethereum Blockchain.
The EthSpring project is an effort to gather and categorize Ethereum learning resources. Browsing through their material you'll learn about the Crypto Philosophy, Peer Protocols, Protocol Theory, Zero-Knowledge-Proofs and more.
Protip: You can also subscribe to the EthSpring Newsletter to get notified when new resources are added.
The Know Ethereum's project goal is to catalog and curate useful content from the Ethereum ecosystem in order to make it easily discoverable by newcomers and experts alike.
Browsing through it you'll find beginner resources to start your learning journey, the most active subreddits, Twitter accounts you might want to follow, useful books, podcasts and more.
The Ethereum Beigepaper is a rewrite of the original, highly technical and mathematical Ethereum Yellowpaper which serves as the technical specification for the EVM, Ethereum's underlying virtual machine.
The Beigepaper's goal is to make the Yellowpaper more accessible to a broader audience by rephrasing the technicalities in a more approachable format.
Reading through it you'll not only learn how Ethereum compares to other projects in the Blockchain space but also how the Ethereum Virtual Machine (EVM) works under the hood.
The Solidity Patterns project is an effort to consolidate the best design- and programming patterns for Smart Contract projects written in Solidity. Browsing through it you'll find patterns from categories such as behavior, security, upgradeability and economics.
Note: While the document refers to Solidity version 0.4 most of the patterns can be applied without any strict version requirement.
Having all the different topics, concepts and language specifics top of mind when working with Solidity can be quite challenging.
The Solidity Cheatsheet distills all of this knowledge into an easy to navigate reference document.
How does Ethereum work, anyway?
The blog post "How does Ethereum work, anyway?" is one of the best reads if you want to get a deep understanding of everything that's involved when interacting with the Ethereum Blockchain. When reading it you'll learn (among other things) how Blocks are created, how logs are stored and how transactions cause state transitions to be carried out by the Ethereum Virtual Machine (EVM). It'll definitely take some time to read through it and grok everything but once you're done you should be in a better position to build decentralized applications on top of Ethereum and the EVM.
The book "Mastering Ethereum", written by Andreas Antonopoulos and Gavin Wood does a great job explaining everything you need to know about the Ethereum Blockchain as well as the Ethereum Virtual Machine (EVM) and Smart Contract development via Solidity and Vyper.
While you should definitely grab a copy at your retailer of choice you can also check out the books source code at their GitHub repository and file Issues / PRs to contribute back.
The Ethereum 101 post by Secureum outlines 101 important facts to know about Ethereum and the Ethereum Virtual Machine (EVM).
Reading through the writeup you'll learn about Ethereum's underlying cryptography, the design of the EVM, EVM Opcodes, Transaction processing, the structure of individual Blocks and more.
Solidity 101 & 201
Browsing through the resources you'll learn Solidity basics, security- and code enhancements as well as various tips and tricks to write more elegant Smart Contracts in Solidity.