The official website of the State of Texas lists all of the 254 counties within the state. The site provides an alphabetical list of counties, along with the county seat for each.
A variety of population data is available for each county, with census details provided by the U.S. Bureau of the Census, which also has a selection map linking to census data for each county. Population details are also available for individual cities in the state. The site provides a link to the online Handbook of Texas website, which gives further and historical information about each of the Texan counties.