The FamilySearch website is operated by The Church of Jesus Christ of Latter-day Saints. Access to their records and content is free with the creation of a free account. Featured resources include a fillable online family tree template, access to their online records database, research tips, online articles, and access to a variety of Wiki pages containing links to an assortment of free and paid online genealogy resources.
The USGenWeb Project, founded in 1996, is a national and global, volunteer-operated organization that provides online resources for genealogical research. All content on this site is available free and no account is required. There are a robust number of resources available through the USGenWeb site which include a national search engine, photos, maps, transactions, historical documents, and various links to other online genealogical research resources. The USGenWeb Project also has a number of subprojects that offer specialized research on topics such as marriage records, court cases, church records, tombstones, obituaries, and immigration records. The content is organized geographically and each state in the United States has a corresponding GenWeb site that provides access to state and local records.
The National Archives was established by Congress in 1934 to preserve and curate important documents relating to the history of the United States government. Many of their records such as census documents, military service records, and immigration records are digitized and available free to the public. Additionally, the National Archives has several pages dedicated to special topics in genealogy research including ethnic heritage resources, tools for starting your research, and information about other online genealogy resources. All content on this site is available free and no account is required.
The National Genealogical Society (NGS), founded in 1903, is a non-profit organization focused on providing education and resources to those interested in researching their family history. This site provides access to free resources for family historians including research tips, fillable templates and charts, video tutorials, access to online articles, links to free genealogy websites, and information about their book lending program. Resources are available for every level of researcher. NGS offers paid memberships, but many of their resources are available for free. There's a lot on this website and it's easy to get disoriented. You'll probably want to start at the appropriately-named, "Getting Started " page.
Genealogy.com is part of the larger family of subscription websites owned by Ancestry.com. This particular link is to a searchable list of posts to the old GenForum. Forums are a great resource because they allow you to search through queries and information posted by other researchers. Though you should seek independent verification of any leads you generate through forum posts, these can be a gold mine when you've hit a "dead end."