Let the evolutionary games begin!
Charles Darwin Survival Game—Thanks to Jeff Drew for suggesting this webtoy. Under the guidance of Darwin, select three variations of the same species and hope they'll possess the genetic gumption to live a million years. Once you've made a choice, you have very few opportunities to reinvigorate the gene pool, making for more of a "toy" then a "game." But the outcome is quick and interesting with beaucoup re-playability.
Bloons Tower Defense 3—More Bloons! Buy monkey towers, dart shooters and other balloon-destroying items to keep the inflated enemies from crossing the finish line. The third in a trilogy of hit games—not to mention the other Bloons titles that are also pop-tastic.