Seed is a simple, open-ended game in which you plant virtual flowers, cross-breed them, and create new varieties. Here's a screenshot after a minute or two of playing:
It gets interesting when users share the "DNA" (not really DNA, but simplified computer code...) for their flowers. In the comments below the game, many users have posted DNA for their creations, and those are good starting points. To use someone else's DNA, copy the DNA to your clipboard, click "show DNA" in the Seed game, then paste in the DNA and click "seed this." (To get your own DNA, drag a flower to the "drag plant here" area and click "show DNA.")
There's also an "evolv-o-matic" button which mutates and cross-breeds your flowers automatically -- leave it running in the background and come back later to see what's happened! Give it a shot if you have a free moment.
I enjoy Seed because it's so open-ended -- you can play around with different flower combinations and watch what happens. It's certainly nothing like real evolution or even plant cross-breeding, but it's still an interesting toy. There are some downsides to this game, as well -- on my Mac, it routinely paused for long periods (thirty seconds) and I thought the browser had crashed. Because of this, I recommend running it in its own browser (one in which you're not doing important work). On Windows it worked fine for me, so maybe it's just a Mac thing.
Did you make a great flower? Share its "DNA" in the comments!