The National Archives has a catalog available online that includes nearly everything in our collection available at the series, file unit or item level. Recently we added a lot of naturalization documents at the item level.
We have also partnered with several groups that include public access to scans of our records.
Two public access partner groups that I use most frequently for family information are:
Family Search is actively scanning material from repositories around the world and relies on volunteers (anyone with internet access) to help them index everything.
The German Genealogical Group is another group that we partner with that has created indexes to some of our collections.
Researchers can access many genealogy websites for free by visiting the research rooms at National Archives locations, including Fold3 and Ancestry. Visit this page on archives.gov for more information.