It totally managed to fly under my radar (or maybe I was just awash with tower defense games), but the game that arguably brought the genre to the masses was "Desktop Tower Defense." It was, if you'll recall, Stephen Totilo's Game of the Year in 2007. (Note: Can't say I agree with Stephen on that one..."BioShock" and "Call of Duty 4" both came out that year, but to each his own!) Game of the Year or no, it was still a great game and has been played by millions of people.
"Desktop Tower Defense Pro" is the next iteration of the series, and it's definitely more of a full-fledged title. There's a full swath of scenarios to master, as well as a sandbox mode where you can change just about every aspect of the gameplay, from creep speed to starting cash.
"Pro" still features the same quaint 2D graphics that made the first game so charming, and there's still the hand-drawn aesthetic that permeates the visual style. At its core, it's really the same game...just more of it, but there's really nothing wrong with that, especially considering it's free.
So, do yourself a favor and stop being productive for an hour or so. Your mind will thank you!
Desktop Tower Defense Pro (Kongregate)