What about a curated list of open-source projects with defined needs, perhaps generated on a month-to-month basis? For example, you guys could comb through GitHub or any other repository and provide users with a top-50 list of projects/descriptions/languages used/etc.
Obviously, plenty of users will continue to contribute bug fixes to the projects they use on a regular basis. But some users might just want an interesting hack that stretches their knowledge and comfort zone - and if it fixes a problem for that project, why not?
I second that idea. And would further suggest highlighting some projects that require roles other than developers, so those projects can get the design, marketing, product, etc help that they're seeking.