Infectonator 2 Absolutely addictive. I'm loving it, and I can't stop playing. I like the amount of different playable zombies, and that you can upgrade them. The strategy of where to use your support items is also a good element.
My only criticism might be that selling things back in the lab would be helpful at times. You could make it an ability that players have to buy though, so it doesn't drop the difficulty too much. And adding some kind of really epic (and epicly expensive) infected, might be fun for people who play long enough to afford it.