Packet Garden www.packetgarden.com
Every time you browse the internet, you create a data trail - a log of where you've been and what you've done. That trail is then used by the websites you visit to target adverts at you, or pepper you with spam and annoying pop-ups. Basically, it's used to annoy the hell out of you. Passively multiplayer games, on the other hand, take that same data trail and empower you to do something worthwhile with it for yourself.
Just like PMOG below, Packet Garden keeps track of the websites you visit throughout each day. But instead of awarding you XP and bonuses for your surfing, it enables you to use the information to create a 3D planet. Every bit of data you upload is represented geographically as a valley, and every bit you download is represented by a mountain. Each time you use a new internet protocol - the web, email, instant messaging, peer-to-peer sharing - a new plant grows. The result is an abstract, Darwinia-esque representation of your computer's journey through the internet, and an explorable world you can travel around as a tiny, ball-shaped avatar. Try it.