For those who dream about covert operations, illicit deals and subverting authority, I have found a place for you!
Guerrilla gardening was initially started in the UK, and it has grown from there. Even Deadmonon has its own platoon of guerrillas, but the effort is still small (outdated website that was last updated last year... I know, its not like I post all the time too!)
It is quite simply gardening where you can. So even if you don't want to join a local group, free crowded plants from their prison in your yard & start planting those patches of dirt. Take a packet of seeds & liberate them. Well... you get the idea, I won't start yelling "FREEDOM!! ala Braveheart now, so you needn't worry. Just get out there & get your city greener! (well, when it gets warmer I suppose)