IMSLP's Community Projects consist of a variety of different projects; scanning of scores, logo removal and uploading, uploading from private sources, and acquisition of scores from various websites. Most people can contribute to most of these projects, although some require certain access. You may even start your own project, just make sure to follow the guidelines.
To participate in this valuable work, simply list your name on a project's page, and start uploading. You will need to follow these simple steps:
New proposals can be explained here. Archives that are bound to get lost should preferentially be chosen.