Ever downloaded a module and noticed several obvious bugs? Often, you can look in the issue queue for a module and find fixes, code patches, and help with these problems. For certain modules, the maintainer of the code may not respond - he or she may no longer be developing with Drupal, may have changed his or her email address, or any number of other factors.
You can take over control of an orphaned project on Drupal.org - there is even a well-defined process for doing so. Read the documentation on Dealing with Abandoned Projects to get an idea of what to do. You may also need to apply for a CVS account, which is covered in the same handbook page.
If you're looking to start a new project (module, theme, translation, or install profile), read documentation on Starting a New Project.