No matter what kind of layout you are planning, there are certain tools that will serve you well in the hobby of model railroading.
Let's start with tools for dealing with the track because we all know that without the track, we have no trains.
First and foremost, since no track you purchase will ever create the layout you envision without some modification, you will need a tool with which to cut the track.
I am a big fan of rail nippers by Xuron. They are easy to use and make nice, clean and accurate cuts.
If you don't care for the nippers, there are saws designed specifically for cutting the track.
No model railroader should be without wire strippers/cutters. Mine are able to cut wire, strip it and can be used with crimp connectors - you know, those little plastic thingies you stick two ends of wire in and crimp using pliers ... or a crimping tool.
Also in your toolbox should be needle-nose pliers. These have a multitude of uses. I recommend a set that has regular needle-nose as well as long, curved and short pliers. 
A good set of precision screwdrivers is a must. They are small enough to be used for the tiny screws on locomotives. Make sure your set is magnetic. Trust me when I tell you that you do not want to go hunting for that tiny little screw you just dropped because your fingers are simply too big to get it back to where it belongs.
That brings me to another tool. It's a little unconventional, but I picked one up several years ago and wouldn't be without it. It looks like a cross between a small screw driver and tweezers.
To use it, you hold it kind of like you would hold a cigarette. Pressing on the top of the picker pushes out and spreads to thin pieces of metal that are bent at a 90-degree angle at the end. You place the head of the screw between the pieces of metal and release the top of the picker.
You can now get that screw anywhere you want it, even in the tightest of spaces. A little turn of the picker will get the screw started at which point you can take over with a regular screw driver.
The above las led me to yet another useful tool - tweezers. Long or short, they, too, have a multitude of uses. From providing delicate assistance when building models, to placing small detail items on the layout without getting glue all over hour fingers and everything else, they are extremely handy to have around.
A good hobby knife is a must as is a package of paint brushes of all sizes.
Other tools I have found useful include:
Hot-wire foam cutter
Heat gun
Airbrush
Soldering iron
Small files
Pin vice (also known as a tiny, hand-operated drill)

Next week, I'll talk about some neat items I have used from around the house to help me with scenery.