PMOG is a multiplayer game that's not actually a game... or is it? Developed by a few programmers who have been fishing around for funding in a few different places, PMOG is actually a Firefox extension that turns web browsing into a "passively multiplayer online game"-- as you surf different web sites, you can gain experience points and currency, and then you can use those to build a virtual empire on a meta-level. Users can build traps and set off on missions on the web, and even wage war over web sites themselves. "Playful annotation of the web at large" is how they describe it-- sounds fun.
It's just recently started up a closed beta, and apparently there's still more testing to go through before the concept is opened up for everyone to jump in on. There are other concerns, too-- privacy, and how they'll make their money (will they require users to visit certain sites or see ads to play?), but hopefully those questions will be answered soon, as they plan to have a demo at GDC (Massively will be there and keep our eyes open for it). The Passively Multiplayer concept itself is definitely an interesting one, though-- it only takes a small, even inconsequential reward system to turn something that's usually boring into something that people get excited about.