/transport - This is your all-purpose 'travel' command. Use it to summon a transport aircraft to carry you to any pre-marked (either default or manually) location. Costs 1500 to use
/auto - Auto-cruise. When in any vehicle, you can use this to travel (albeit in a straight line, so it's not usable in cities) to any bookmarked location. Advisable in certain situations, but generally it's better to fly.
/taxi - If you're in town and not near a hangar, then it's best to use this. It rents you a little car to wander around town in, as otherwise it'd be a very long walk to your destination. This command only works in cities. Costs 650 to use.
/getoff - Get out of your current vehicle. If you're in a taxi, it just deletes it from existance. If you're in a proper vehicle, it ejects you. Generally speaking, you're better off parking vehicles in a hangar (double-click on the hangar building) than leaving them out in the open.
/mark - Bookmark your curent location, for future use via the /transport and /auto commands. It's advisable to put down a bookmark at every hangar you find, even if you already have a fairly close-by default bookmark. This cuts down travel time dramatically, and allows you to go anywhere you've been before simply by parking at a hangar, transporting to your new location and getting back into your vehicle.
/trade - Target a friendly unit (click on them in peace mode - a green hand pointer should appear on them) and type this to open the trade window. You can drag and drop anything from your inventory (backpack or container), or any nearby objects/vehicles you own into your trade panel.
/transfer - This is good for transferring Mobile Suits and other vehicles from player to player. Target a friendly unit (same way as in /trade), and type this to open the transfer window. Then drag and drop your MS/vehicle into the window and press OK, and the thing switches owners.