Much of the land has gone to homesteaders settling the West, of course. But a lot of other land grants (land patents, as they're often called) have happened as well...to veterans as a reward for military service; to Indian tribes as territory for reservations; to local jurisdictions as parks, townships, mineral rights transfers, just to name a few.
All told, there are millions of land records available online through the Bureau of Land Management. Most of these are actual images of land patents, though there's also a lot of index information, along with maps, and even an occasional census or two of the inhabitants of a jurisdiction being transferred.
David Crockett's land grant. Is that you, Davy?
You can search federal land patents at BLM. Use the Standard tab to search all states at once (the all states option is at the bottom of the pull-down list).
You can also search BLM records for land survey plats. There's more information here than you might suspect. Survey maps, of course, but also voluminous reports about the area under survey, including, sometimes, the inhabitants. Search and retrieval is awkward, but there's a lot here that might make it worthwhile.
Land Status Records are available for several Western states -- Colorado, Idaho, Montana and the Dakotas. These are official recordings of who has what rights when federal and local jurisdictions overlap. Of limited genealogy value, but there is some interesting history here, for those with family in the areas covered by the records.
Visit the main page of Free Genealogy Tools for more, umm, free genealogical tools.
And don't forget to also check for your family history at Ancestry.com and NewspaperArchive.com. These are subscription databases, but they are among the most powerful research tools available for looking into family roots.