Tennessee State Library and Archives
This is the website for the Tennessee state library and archives. They have an extensive manuscript and microfilm collection which a searchable catalog to locate needed records. They also offer research services. The downside to the website is that most of the electronic databases and indexes you can search require you to be a resident of the state of Tennessee.
TN GenWeb Project
They have many searchable databases on both the state and county level. There is no charge to use the site.
One of the interesting things to remember about Tennessee is its origin. A good portion of it came from territory that was once part of North Carolina. If you're looking for really early land records (before Tennessee became a state in 1796) you might want to try looking in North Carolina. Many Revolutionary war veterans received bounty land in the area now known as Tennessee so you might want to check bounty land warrants.