Total Mayhem takes Total Annihilation, bugfixes and extends it the way Cavedog might have. Early game pace is quicker, many new units added (longtime players may recognise some 3rd-party units), including a whole new 3rd tech level tier. Existing and added units have been bugfixed, rebalanced, re-modeled, re-textured and re-rendered to equal or exceed the standard of the original TA units.
New version at last! removes a lot of units but adds a lot of polish... and also adds the Spider pack 2012 and the Tortoise Pack, and metal extractor upgrades (for both land and underwater versions). Economy has changed to a constant metal output for construction units and factories, to make managing your metal much easier.
There have also been improvements to make aircraft more useful, by making it harder for non-anti-aircraft units to hit and damage aircraft.
For more detail (and to post feedback if you like), please visit this TA Universe forum topic: