I'm working on an open source pure functional programming language called Lambda Zero that aims to be a simple and elegant base language from which to derive more advanced languages and eventually a foundation of mathematics. Like Haskell, it is basically just the lambda calculus with a bunch of syntactic sugars, so it's more a process of discovering the foundations of computation than inventing an arbitrary new language.

There is currently a bootstrap interpreter written in C that is under 2000 lines of code, a self-interpreter (~1200 lines), and a static type inferencer written in Lambda Zero (~400 lines). I'm currently implementing pattern matching lambdas and algebraic data types and I have a long roadmap of things to do. It would be great if someone was interested in writing an optimizing compiler for Lambda Zero in Lambda Zero.

https://github.com/clark800/lambda-zero

Open worm is attempting to simulate C. elegans: a very simple multi-cellular organism completely. Atom by atom, cell by cell, neuron by neuron. Really cool project.

http://openworm.org/ https://github.com/openworm

Airflow is a fun one. Currently in incubator phase with Apache so there is plenty of stuff to do especially if you like working with Python, Docker, JavaScript, web apps, etc.

https://github.com/apache/incubator-airflow

I co-maintain CellProfiler. Contributions are always welcome!

https://github.com/CellProfiler/CellProfiler

The mooltipass project actually is looking for contributors for its next gen device! If you'd like to work on hardware (low power measurements, rf design), firmware (https://mooltipass.github.io/minible/) or software (https://github.com/mooltipass/moolticute) do not hesitate to drop us a message :).

Great question. Anything that furthers the field and does not risk becoming irrelevant in 6 months would be an excellent bet.

+ Utilities for computers and disks are always useful and don't need much maintenance for long-term function/feasibility.

+ web frameworks that make web designing/dev easier (clojure[script], rails, python+django are all popular choices)

+ Look for a tool you commonly use or a service that's commonly requested.

WordPress may be old news but the new Gutenberg editor has the potential to disrupt the most popular CMS on earth. Helping make this a success isnt sexy but it is meaningful.

I think the best and most exciting projects to contribute to are the ones you actually use and want to help make better.

VTK - Visualization ToolKit - strongly recommended if you want to work on a widely used, probably defacto, scientific visualization library, a great 3D domain to work on IMHO.

Backed by a ton of resources, documentation and excellent community; that will be fun.

https://www.aosabook.org/en/vtk.html, is a good start.

I've always loved blender, it is open source and I'm sure they would love any contributions.