Desktop Plant lets Windows users keep and care for a plant, on the computer desktop. It looks like they have a Dogz-like pricing model at $10/plant (to reinforce the idea that you really ought to take care of your plant), which in high school I thought was aggrivatingly dumb but today I think is kind of cute. Especially cute in this regard is the "automatic care system" add-on you can buy for $25 that babysits your plant for you. You can download a demo version for free to see what it's like. (Thanks, of course, to Zannah.)