Tax records provide us with yet one more way to find our ancestors, even if we are unable to find them in the census. The census taker may have missed our ancestor but the tax collector most likely did find them. Remember as you search, that our ancestors paid various kinds of taxes, just as we do today. While there is not enough space here to go over all the different types of taxes they may have paid, you may want to consult a book such as The Genealogist's Guide to Researching Tax Records by Carol C. Darrow and Susan Winchester, to better understand the taxes through history that our ancestor's would have had to pay.
Where can you find tax records? Start with the usual places such as the Family History Library Catalog. Try conducting either a keyword search or a search on the locality where your ancestor lived. If you search on the locality, once you have the results for that locality, look for a "Taxation" link. Many of these records will be available on microfilm and you can borrow them from your local FamilySearch Center (formerly Family History Center). You may also want to peruse the FamilySearch Wiki for articles about tax records. There are over 4,000 articles about tax records, including, Virginia Taxation; Tax Laws and Records; Introduction to Tax Records in England; and Beyond Parish Registers:A Case Study.
Many subscription sites have tax records. One example can be found in the Ancestry.com record group Tax, Criminal, Land and Wills. One of the databases Ancestry has is the U.S. IRS Tax Assessment Lists 1862-1918. Through the Ancestry Wiki, you can learn more about researching tax records by searching on The Source by Loretto Dennis Szucs and Sandra Hargreaves Luebking, or learn about a locality's tax records by searching in The Red Book by Alice Eichholz.
Cyndi's List contains a list of locality-specific websites with tax records. You can check this out on Cyndi's List-Taxes.
As you research your ancestor, remember to identify possible tax records that may include your ancestor's name. This will provide you with one more way to learn about your ancestor in place and time, as well as learn more about their life.