If you are working on a project, you sure use open source projects, libraries and frameworks. All these stuff helps you build something amazing and do it faster. And also you know what a good feeling it is, when someone write a good review or give your fan page like. So do the same.
Did you find some GitHub repo interesting? Did it help you? Give it a star! Don't be shy. The developer behind it will know his work is meaningful and will be motivated to make it even better or create another project that might help you.
You have a problem and you found an article or blog post that solves it? Leave a comment. It is not that hard. It does not need to be long and nice, just "Helped me a lot, thank you.". See?
What a small thing to do and what a big thing it can leave behind.