Last updated November 13, 2012.
When Drupal.org was running CVS, there was a global "sandboxes" directory where many users uploaded experimental code. These sandboxes were not migrated over to Git, so if you want the code that was in these, you need to do so manually. Here are the steps.
1. Export a copy of your sandbox from CVS ("export" rather than "checkout" so you don't get pointless CVS folders):
cvs -d:pserver:anonymous:firstname.lastname@example.org:/cvs/drupal-contrib export -d mysandbox -r HEAD contributions/sandbox/[your-username]
2. For each project you'd like to bring in as a "proper" project, go to Create project, and create a project checked off as a "Sandbox". Navigate to its "Version control" tab.
3. Back on the file system, move into the project you'd like to convert to Git, and use the following commands to move it into Git instead:
# Get the files into Git.
git add .
# In the text editor that comes up, enter a long description of what the project does, and save.
# Get them onto Drupal.org.
# Note: you can copy/paste this line from your project's "Version control" tab.
git remote add origin [your-username]@git.drupal.org:/sandbox/[your-username]/[your-project-id].git
git push origin master