Updated for CB4, ArGL patch.
100% serious. Green Lantern. Not a newly coming out one either, the blaster. The mage. But how? Why? Simple. Your team needs power damage, but not Flash. You already have Shazam in lane. Or maybe you just really like Green Lantern. Whatever your reason is, it's ok, because it turns out, he's a pretty good jungler. His passive triggers from creep deaths (even though it reads kills/assists), and power damage builds aren't all that costly early.
Items: Marauder's Ring 1, Marauder's Shield 1, Flask. Sell Marauder's Malarkey ASAP for Two Face's Coin. Assuming you aren't dealing with heavy counter jungling, build glass cannon. However, you are somewhat likely going to have to deal with lots of counter jungling, so the build I've layed out is rushing the new anti-dead item, Neron's Contract, and ending with Ra's al Ghul's Robe. The rest of the build is straight forward, and the Atlantean Royal Seal could be dropped for something else. I like it, however. It gives you AOE slow on your rockets, and a spammable single target slow via Q, usable between W activations.
Jungling: Stand in the middle of a camp so E hits everything. E, auto, E, auto, E, auto, Q. Later, with higher ranks of abilities and lower cooldowns, QWEEEQ. His jungle clear is really easy, and once you have some power damage, stealing enemy camps is quite quick. EEEQ once you have Two Face's and Radeon Shard is enough to kill any camp other than the 3 minute respawn. Your only concern in the jungle is the very real possibility of a Nightmare Batman or the like running in and ruining your day. You generally aren't going to win a 1v1 against an Assassin, so top jungle is preferred, as it's the safest and easiest to keep covered, to avoid wandering into the waiting Maw of a bat or a cat.
Teamfights: You may been jungling all game, but you're still just flimsy ol' Green Lantern. Stay safe until the team fight is initiated, then do your thing. You should never try and be the one initiating. You will very likely die for it. Try and be sure to hit as many people as possible with the stun effect of your plane, though it will likely be 2 at the most. I know it's tempting to rush in and start throwing your spells at the enemies flimsy carries that need to hide behind the tank wall, but uh... you are ALSO a flimsy carry who needs to be hiding. Deal with the bruisers/enforcers waddling into you before you give chase.
Abilities: E is better for clearing than Q, but you don't need more than 2 points in it for that. Passive is tempting to level before W, but the duration on W is extended, and the frequently overlooked aspect of his W is that hitting people with abilities while they are under the effect of W lowers the cooldown on constrict. More uptime on W means more hits during W means more W activations means more abilities into... you get what I mean. It's a very potent slow. possibly even worth maxing before E against melee heavy teams, I'm not sure on that yet.