This website holds a directory of some of the potential projects for Google Summer of Code 2019 that students can apply to. This is an unofficial website that is not affiliated with Google or the Google Summer of Code. (Although hopefully it has their blessing.) It was created by Yaron Koren, who has been a longtime GSoC mentor, via the Wikimedia Foundation (and hopes to be one again this year).
To add your own project ideas, or ideas from your own organization, you can use this input to enter a project name; clicking "Create or edit" will take you to a form for adding some structured information about that project:
Go here to see a drill-down listing of all project ideas submitted so far, from across all the organizations:
This website runs on MediaWiki plus a few "Enterprise MediaWiki" extensions, like Page Forms and Cargo. Since this is a wiki, you can create any pages you want, in addition to project information, although this wiki is not really intended for any other types of data at the moment.
Note to students: this website is intended to be a helpful way to find projects that hopefully match your interests and skills. However, it is not an ideal place to contact the mentors of these projects; if you write a comment on a talk page, there is no guarantee that the mentors - or anyone else - will read it. Instead, it is best to communicate with mentors using whatever communication mechanisms those mentors or organizations provide.