My primary source has been Ancestry.com. Information I have found in their databases is cited with Ancestry.com shown as the repository. Full access requires a subscription, however you may be able to use the databases through your library's Ancestry Library Edition subscription. Member trees that I have cited were only available to me through guest passes issued to me by the tree owners. They are otherwise available to individual Ancestry.com subscribers.
HeritageQuest is a great resource for census records for persons without access to Ancestry.com. It is a subscription service available for free at most public libraries. You will probably need a library card to get the internet address, login, and password from your library. After that, access is available from home. I have used it's "Search Books" feature to get biographies and regional histories. The Genealogy of Henry and Mary Brosius can be viewed or downloaded from there also. It is a 1928 genealogy study that I found very useful.
Research done by others was also available to me through trees sent to me by cousins in my Brosius and George lines. I got my start on the families descending from Anna Lammes (Families with the surnames of Wall, Neumann, Koch, Bunte, Kraatz, Shoults) from two books written by cousins of my husband: LaVerne Harrell Clark's The Bunte Family History and Dolores Fasel Bozeman's The Wall Family, 1849-1996
I referred to the family trees on RootsWeb.com World Connect Project a great deal. All these trees are cited with enough information to access them. Copy the tree code following the word Gedcom in my citation and paste it into the "jump to a specific database" box at RootsWeb.
Texas has made its databases for marriage and divorce available for the years 1966-2013 at Marriage License Application Indexes. The divorce records for the years 1968-2013 are available at Report of Divorce Indexes. Databases retrieved from either site require a program such as Excel to access them. I found a much easier solution for searching through the Texas marriage and divorce records. Texas Marriages Search Engine and Texas Divorces Search Engine has those databases in an easy to search format.
A great deal of research information is searchable at The National Archives. I used it primarily for military records.
Death certificates provide a great deal of information. The death date and place, the birth date and place according to the informant (not always accurate), last residence, military service, occupation, parents, cause of death, burial location. Sometimes the name of the informant provides a clue to a family relative or even the marriage of a daughter. Two websites provided all the death certificates cited in my family tree. Missouri Digital Heritage and Family Search Record Search Family Search was most valuable for Texas death certificates from 1890 to 1976. Occasionally I found death information, but not death certificates for other states using the search there.
Birth and death dates for anyone who died after 1962 in the United States are very likely available in the Social Security Death Index.
Tombstones are great sources for birth and death dates. Some have carvings indicating "wife of". "son of", or "daughter of", which are very helpful. My cousin, Gene Homer George, who amassed most of the George tree I started with has a 3 generation family tree carved into his tombstone. Placement of the graves in a cemetery can suggest connections between family members also. Some FindAGrave memorials include plot numbers which are clues to placement and therefore relationships for those of us that can't visit the actual cemetery. FindAGrave.com memorials often include a tombstone picture, sometimes a copy of the obituary, and links to other family members. I have found that my ancestors were very consistent in their use of cemeteries. I have found hundreds by searching surnames within the same cemetery in Mississippi, Alabama, Missouri, Colorado, and Texas.
I have found some family Bible records that gave evidence to births, marriages, and deaths. In every case so far, I have not seen the actual Bible, but only images taken from the Bible pages. Some of the records were typed copies of records from family Bible pages.