Here's what I've heard about it so far...

Please post anything new from the magazine... I'm still trying to find the issue somewhere! lol.

New Vehicles-
Prowler: Basically a brute version of the Spectre.
Hornet: UNSC fighter similiar to the Banshee, but can hold two passengers on the sides.
Elephant: UNSC GIANT ****ING THING. Only known to be usable in Sandtrap as of now. It's pretty much a drivable base. Imagine a Jawa Sandcrawler, except with turrets, you can spawn in it, your flag is in it, and it is big enough to hold a Scorpion, a Warthog and two Mongooses.

Weapons-
Gravity hammer: Giant brute hammer, makes a big shockwave and makes vehicle swiss cheese.
Mauler: Dual weildable Brute shotgun.

Equipment-
Radar Jammer: Creates tons of fake radar blips on enemy motion trackers.
Personal sheild: Like a bubble shield, but it only faces one way. The person who deploys it can shoot out, nobody can shoot through.
Regenerator: Regenerates your sheilds. Basically like Shadowrun's "tree of life".
Flare: Pretty much just a flashbang.

Levels-
Guardian: Jungle redo of Lockout.

Features-
Forge: At any point you can become a monitor (like guilty spark) and go around and spawn things. Anything, weapons, vehicle, objects, spawn points, etc. You can then save your creation and upload it to bungie.net for other people to download. You can also have many people playing forge at once, becoming monitors and doing all kinds of wacky shenanigans.

Since it won't let me edit my post, I'm forced to make a double post. But I went out and bought the EGM today and was very amazed by what I saw.

Endless customization options. Endless customization options combined with endless map configurations in Forge. Endless playability in campaign mode. Endless re playability in co-op campaign. The possibilities are...well, you know. And with map sharing, new-map downloads, leader boards for metagame high scores... Halo 3 seems primed to shatter all records again. Oh, no, we're not talking about sales figures here. We're talking sleep deprivation.

Here's for everyone who is forgetting just how much play value is in Halo 3.

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-Campaign-

4 player Online/system link Co-op.
2 Player Split Screen Co-op
(You can have a split screen team play online with others)

"Campaign Scoring," a metagame that effectively turns the campaign mode into a competitive challenge similar to multiplayer. This metagame, which can be played solo or in co-op, tracks each player's performance and assigns points to each kill; the more impressive the performance, the higher the score. Players also earn medals for certain actions, just like in multiplayer, and lose points for particularly bone-headed maneuvers like killing their co-op partner. At the same time, you can boost your points by finding and activating hidden skulls, each of which unlocks a different aspect of the metagame.

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Multiplayer, Map Editor, and More!

Custom Spartans AND Elites
Available in Many colors with options for shoulder attachments, helmets, and chest pieces.

Forge-
You play the role as a Monitor, those floaty-camera-ball A.I. guys (343 Guilty Spark)
You fly around the maps from within (Like you're actually playing the game) and place objects, grab and move them around, or delete them. The items range from crates, barricades, and spawn/capture points to any weapon, equipment, or vehicle in the game. and all can be modified as far as ammo count, respawn times and other simple parameters. At any time, you can push up on the D-pad and turn your Monitor into your normal multiplayer character model and start playing the level you're modifying. This allows you to instantly test your creations. You can do all this simultaneously with your friends on Xbox Live, system link, or split screen.

Once your done fiddling with a map in Forge, you can save it and pass it on to your friends thought Halo 3's file share to use in normal custom games. Bungie.net will serve as a hub to find user-created stages as well. They'll have an area for Bungie-recommended maps that are made by the community, and they'll have a way for you to search out others. Once you mark a level for download on Bungie.net, it'll auto-matically transfer to your Xbox 360 hard drive the next time you long in to xbox live and load up Halo 3.

New Stuff:
Gravity Hammer- It will do a short lunge like the sword does, but mostly you use it to do a area-effect damage

Hornet- Marine aircraft, carries two gunner passengers and 1 pilot w/ machine gun.

Radar Jammer- Tricks enemies with false radar images

Personal Shield- Stationary version of Jackal shield, can place anywhere player selects.

Regenerator- It's green field instantly recharges players shield's. (like the tree of life in Shadowrun)

Mauler- Brute Shotgun that is duel-wielding, but less powerful than the human shotgun.

Flare- Basically like a flash bang grenade.

Prowler- This brute craft holds a driver, a man on a forward mounted turret, two passengers on the side, and looks like a jet turbine engine on a sled.

Fourth Grenade- (Never seen in action, but is there)

http://www.starwars.com/databank/vehicle/sandcrawler/img/movie_bg.jpg
Elephant- Looks like a Jawa-Sand crawler from star wars. It's a human recovery and mobile operations vehicle. This is essentially a mobile base. It is big enough for a dozen-plus players, have a horn that sounds like it should be on the Titanic. It's drivable, and has a mounted turret, can respawn troops, holds your home flag, and is the return point for the enemy flag.