I contribute to a lots of opensource applications and libraries, primarily in the Erlang ecosystem.

I'm one of the maintainers of epgsql - the most popular PostgreSQL driver for Erlang.
Also, active contributor to gen_smtp - library that allows you to send and receive email from Erlang apps.
Made several contributions to Erlang/OTP and countless amount of smaller apps and libraries.

Some of my own projects include

• mtproto_proxy - Erlang implementation of Telegram proxy server that allows to bypass Telegram messenger bans in some countries like Iran and Russia.
• eplaypen - web-site that allows execution of arbitrary Erlang programs from your browser

I do most of my opensource work at my free time as a hobby, as a way to support Erlang platform (because I think it's great) and as a way to pay back to the opensource community.