Puppy Games' Revenge of the Titans →
Puppy Games (makers of the fantastic Titan Attacks, which I played the hell out of) have recently released a beta of their upcoming game Revenge of the Titans, a kind of tower defense and RTS mashup with a distinct Puppy Games flavour.
The beginning levels are short, slow and boring but things ramp up quickly and soon there is a stampede of hopping space-invader-esque aliens rallying to chew on your towers like a pack of ravenous puppies.
Unlike most tower defense games there is not a fixed route the aliens will travel along, instead they head in only a vague direction (sometimes using roads to travel vast empty spaces more quickly) and will disperse as they reach your outpost, sometimes swarming around small defense outposts.
Building up your defenses requires resources, resources that can be acquired from alien kills, random pickups (think Command & Conquer crates) but are found most abundantly by harvesting minerals. After each mission you pay a visit to the R&D department to drop your dollars on researching new ways of making your life better, ranging from automating tedious manual tasks (reloading, collecting harvested minerals, etc.) to improving firepower, to learning about your enemy (and thus making your weapons more potent.)
The music is fantastically atmospheric and the visuals are simple but beautiful. As with every other Puppy Games product, the game is available for Windows, Mac and Linux and is definitely worth playing, even if only to wile away a few hours of the work day.