btcd is an alternative full node bitcoin implementation written in Go (golang).
This project is currently under active development and is in a Beta state. It is extremely stable and has been in production use since October 2013.
It properly downloads, validates, and serves the block chain using the exact rules (including consensus bugs) for block acceptance as Bitcoin Core. We have taken great care to avoid btcd causing a fork to the block chain. It includes a full block validation testing framework which contains all of the ‘official’ block acceptance tests (and some additional ones) that is run on every pull request to help ensure it properly follows consensus. Also, it passes all of the JSON test data in the Bitcoin Core code.
It also properly relays newly mined blocks, maintains a transaction pool, and relays individual transactions that have not yet made it into a block. It ensures all individual transactions admitted to the pool follow the rules required by the block chain and also includes more strict checks which filter transactions based on miner requirements (“standard” transactions).
One key difference between btcd and Bitcoin Core is that btcd does NOT include wallet functionality and this was a very intentional design decision. See the blog entry here for more details. This means you can’t actually make or receive payments directly with btcd. That functionality is provided by the btcwallet and Paymetheus (Windows-only) projects which are both under active development.