The "EtherVM" project is a collection of resources for everything related to the Ethereum Virtual Machine (EVM).
Browsing through it you'll find an opcode reference, documentation about the EVM stack, info about the EVM storage- and memory layout and more.
Protip: There's also an Online Solidity Decompiler you can use to decompile Solidity bytecode into more readable Solidity-like source code.
- EtherVM