Bitcoin

Bitcoin was the first ever blockchain network to be established successfully. It was written in C++. It is now a global chain and is being used every day in the form of a cryptocurrency. Bitcoin is a very popular opensource, so you can contribute to it for free! Although, you will need to learn the working of bitcoin, we are here to help you out. Here are a few resources that will help you out:

There are a few sources that you can avail to learn bitcoin -

1. CS-120: Bitcoin for Developers
2. PRDV151: Bitcoin for Everybody
3. Learning @ Chaincode
4. Bitcoin
5. Bitcoin Github - Nothing better than the open source code of Bitcoin itself to learn about Bitcoin
6. Bitcoin Resources - Bitcoin has provided several resources to learn about bitcoin and related stuff on it's own website in this section.
7. Read the Bitcoin Whitepaper (link in White Papers section)

The books listed in the books section will also prove beneficial to learn about bitcoin.

If you want to contribute hugely in the Bitcoin community, keep checking for the Summer of Bitcoin. It is a huge platform for people who want to contribute in bitcoin.