From Popcap comes this cute, strange and addictive, action strategy game called Insaniquarium (online version requires Internet Explorer). Originally developed by Flying Bear Entertainment, Insaniquarium won the Innovation in Game Design award at the 2002 Independent Games Festival (IGF).
You begin by feeding fish to keep them alive, and collect the coinage they drop to buy eggs that complete each level. The eggs buy you upgraded fish that help you in your quest. It all seems realtively believable until the aliens arrive, the equivalent to Godzilla in the classic SimCity. Crazy fun.
Play Insaniquarium (Sorry, Windows only.)
If you enjoy the online version, the downloadable deluxe version offers 20 all new levels, and 4 different game types: Adventure, Timed, Challenge, and Virtual Tank! You can even name your fish in a custom tank, and create screen savers, too.