Property deeds are the primary records of real estate transactions, and land conveyances are a key tool for researching New York family histories. They can reveal a wealth of information, even those not directly related to real property ownership, including the names of spouses and children who owned the property before or after the sale, the names of trustees, executors, and guardians for minors; and details of the transaction itself.
The City of New York maintains an online database called ACRIS that provides access to land documents and data dating back to 1966. Prior to that date, many documents relating to NYC property can be found at the borough offices of the City Register. A list of these offices can be found here.
Many county courts also have their own land records, and many of these are available on microfilm at the Family History Library. Some of these counties have published indexes, which are also available on microfilm. The State Archives has a number of publications that include various types of colonial land grants and patents, and the microfilm for these records is distributed by the New York Genealogical and Biographical Society.
Some of the land records in the New York land transactions State Archives are indexed by name of grantor (buyer) or grantee (seller) and by year. For example, the Secretary of State’s Military Patents and Abstracts, 1764-1846, ten volumes, includes the names of those who bought military bounty land in the central part of the state (present-day Erie, Chautauqua, Cattaraugus, and some of Niagara County) as well as the reason for the purchase.
Other collections of land conveyances are available on the Internet. For instance, the Center for Brooklyn History has a collection of abstracted land conveyances. Each abstract typically has a schematic map of the land, names of the grantor and grantee, and information about the transaction. You can use the abstracts to find full recordings of the conveyance at 210 Joralemon Street, Brooklyn.
The military bounty lands purchased by the state for Revolutionary War veterans opened up large tracts of land in central New York, and most soldiers sold their allotments rather than settled on them. A summary of the Bounty Land Purchase and Morris’ Reserve, 1791-1835, by Orsamus Turner can be found in The Balloting Book, and Other Documents Relating to Military Bounty Land in the State of New York (Albany, N.Y: Packard & Van Benthuysen, 1825).
Following the Treaty of Canandaigua in 1794 land was transferred from the Oneida, Onondaga and Cayuga Nations to settlers and land speculators. This opened up a great deal of the Finger Lakes area, including present-day Monroe, Wayne, and Franklin Counties. The Holland Land Company purchased the Dutch colonial land in these counties and surveyed it, and its records are on microfilm at the Family History Library (film 974.7 H2t). You can find a listing of films for this collection here. Similarly, you can find a listing of other sets of Holland Land Company records at The City of Amsterdam.