If there's one thing I love wasting time with on on my HTC EVO 4G, it's tower defense games. While I'm more a fan of games where you create a deadly path of destruction for the enemy as opposed to setting up defenses next to a path, sometimes that just works for me, which is why I like Swamp Defense Lite.
The object of the game is to defend as long as possible with your neighbors/kinfolk as wave after wave of swamp monsters come inland, intent on whatever it is swamp monsters are intent on (probably something disgusting and unsanitary). With harmonica music playing in the background, characters that squirt denture adhesive to slow the monsters, and others that chuck pots and pans, the game has a quirky appeal that doesn't wear off too quickly.
The only real complaint I have is that the fast-forward requires the button being held down to go faster. This gets old after a few hundred waves of swamp monsters have met their deaths and your thumb is sweating on your warm EVO.
The premium version is available for $1.20 in the Android Market, or you can save $0.21 by purchasing via Amazon's Appstore.