Harvest: Massive Encounter is a survival-based real-time strategy game with several modes of play that lend a free-form tower defense feel to the experience. You play the humans defending an expanding plot of land against swarms (and I do mean swarms) of alien UFOs, mechanized bots and other baddies. It's an extremely frantic game that's usually more nerve-wrecking than brain-stretching.
Harvest is an upcoming game by Oxeye Game Studio currently in open beta. It is survival-based game that combines elements from tower defense and other real time strategy games. Those familiar with either of these types of games should feel right at home.