If you're playing Halo 3, it's great to know where the important locations on the various online maps are located. Here is a description of each map, along with what to keep your eyes open for.
Last Resort is an interesting industrial map with a large, turning gear and beachfront property. There are NUMEROUS overlook spots which are great for sniping.
There's an active camo located in the center of the spinning wheel. Go up into the center area of the wheel and jump down into one of the three "hubs" or open areas which lead out to the actual wing of the machine. As the wheel rotates down to the bottom position, you can hop in to get the active camo.
Every time I see Sandtrap I think of Dune. I start making comments about sandworms and Shai Hulud and "wormsign". There are even two gigantic harvesters! They move very slowly, though. They have two turrets - one of them is detachable - and a mongoose in each one.
A great pasttime on this map is to run people over with the vehicles.
The Valhalla map involves a northern landscape - pine trees, lots of rocks and short green grass, along with a few mountain streams and a waterfall. There are two main bases made of grey concrete. Each base has a gravlift in it which launches you in the direction of the enemy base.
This is a good map to really practice and learn, because the streams and rocks make roadblocks in the way of fast running and driving. The more you really know this map, the better you'll do.