Image analysis [1] [2]. Statistical packages. Rewriting old, but powerful algorithms into modern languages. APIs to access databases such as Entrez, Uniprot, etc. Stupid parsers to parse still-used god-awful ancient data-formats into standard JSON/XML (looking at you genbank and pdb).

Here are a couple of great biological scientific topics that are used by a lot of researchers that could always use some help if you find one that is active and to your liking:

- https://github.com/search?utf8=%E2%9C%93&q=ImageJ

- https://github.com/search?utf8=%E2%9C%93&q=Entrez&type=Repos...

- https://github.com/search?utf8=%E2%9C%93&q=Uniprot&type=Repo...

- https://github.com/search?utf8=%E2%9C%93&q=particle+tracking...

- https://github.com/search?utf8=%E2%9C%93&q=genbank&type=Repo...

[1] http://fiji.sc/

[2] http://bigwww.epfl.ch/sage/software.html?topic=bio