If you want to learn about blockchain technology through building one (albeit a simplified one!), here are some good articles:

• hackernoon.com/learn-blockchains-by-building-one-… (Python)
• medium.com/@lhartikk/a-blockchain-in-200-lines-of… (JavaScript)

As for building an application that uses a blockchain, there are so many emerging options but I'd say the best for 'getting started' is definitely Ethereum. It is (relatively) mature and has a large community behind it.

If you're looking for an introduction to writing decentralised apps with Ethereum smart contracts I'm writing a blog series on exactly that! Check it out: blog.jaak.io/crossing-over-to-web3-an-introductio…

I personally prefer Ganache and use it along with Truffle framework.

According to me these are some of the good Open Source Blockchains to track:

• Ethereum (Obviously)
• DASH
• Monero
• NEM
• Stratis

Additionally, you can find a bunch of blockchain based projects on GitHub. I have invited George and Luke to answer this question. Please add any other important blockchains that are good for dev and learning. :)

Thanks for the question!