Help a family or organization with a website (Facebook page, etc.) featuring missing persons. Nonprofits often have significant IT/website needs.
Podcasts are an excellent - and growing - method of educating the public about missing persons.
Nonprofits almost always need volunteers for a wide range of activities.
Some families and organizations organize periodic searches for long-missing persons. Joining such searches helps the family, keeps the story alive, and might bring closure.
In older cases, records may be in boxes in someone's basement. If so, they'll need to be sorted, scanned, and returned.
Help others hone their computer / research skills, take them to the library, etc.