If the skill you want to try out is more exotic, then try SourceForge. You may find a project that needs help with the very thing you just read about.

Contributing to an existing open source project is definately a great way to improve your programming skills. Browsing through existing projects can also give you an idea of which areas need more projects started (accounting software), and which don't (php content management systems).

Keep in mind, ever since SourceForge changed its terms of service and privacy statement some people have been encouraging projects to break free from SourceForge.net and to make use of other systems. Just something to consider.