Mega Mash - I'm apologizing in advance for ruining your productivity for the rest of the day. This is one of the most imaginative games I've played in a while now. Not only is it a great gameplay idea, but I love the way it's presented, as a broken 8-bit multi-game cartridge. Basically, the idea is that the cartridge is producing read errors, and as a result all the games are glitching into each other. This becomes the basis for some great gameplay. Plus, they really hit some very familiar genres with only occasional and slight copyright infringement

http://www.duelinganalogs.com/game/infectonator-2/ - has the usual progression and powerups. Vaguely amusing zombies available :-)