There are baking and mixing tools, which most of us are familiar with, as well as the basic decorating tools like a spatula and piping bags. Beyond that, there are some not-so-basic tools, that tend to be more expensive, but allow the serious cake artist to really expand the boundaries of cake! There are also specialized tools for working with fondant, gumpaste, pastillage and cold porcelain. And then there are tools that were designed for some other use completely (but work great anyhow!)
1. Baking and Mixing
At the very least, you need a bowl, a spoon and a pan and oven to bake it. Of course, nobody would bake without measuring cups & spoons anymore (my grandmother use to make the best bread and cakes in the world, measuing only with her hands and eyes... juggling the quantities by how full the bowl looked, or how the dough felt!)
Of course, Granny never had a Kitchenaid! Perhaps my favourite tool, I really love my KA mixers! The size you choose will depend what type of baking you do (the 6qt pro is a great idea if you're making wedding cakes ar a lot of smaller cakes at the same time, but it may be too large if all your cakes are small birthday cakes!) and the power will depend on your budget, and shopping savvy! Each size is available in a couple of different power levels, so make sure you're comparing the same model when price shopping (the higher the wattage on the motor, the stronger your mixer will be, and usually more expensive!) Of course, if you are going into business, you're going to need a Hobart, but be prepared.. these bad boys are a serious investment (cash and floor space!)
My favourite bowls are still a set of stainless steel... I can heat them over a bain-marie to melt chocolate or start a Swiss meringue buttercream, they chill quickly when required, and most importantly, they STACK! In a small kitchen space, nested bowls are the only thing that makes sense. You can buy many different sizes, and easily get multiples of any size you need a lot. They're great for lining up your ingredients before cooking, don't stain when mixing icing colours and can be tossed in the dishwasher! Also very important, they can be very thouroughly cleaned (important both for general food safety, and for making royal icing, where even the tiniest trace of grease can ruin a whole batch!) I have a few good, heavy duty ones as well as a bunch from the dollar store... I like both equally. If I dent or damage the dollar store ones, it's no big deal!
As for the spoon... I have becone a big fan of the silicone spatulas which have become quite redily available. Tons of different sizes and shapes, they're great for hot and cold (no melting the end in a hot pot!) The only real drawback is the fact that, like plastics, the silicone hold onto grease and is difficult to get 100% grease-free for royal icing (so you have a choice... keep 1 spatula for RI only so it stay grease-free, or use a metal spatula or spoon when working with royal.)
Now when it comes to measuring devices, accuracy pays! Anyone can make a cake using "close" measurements and not worry about accuracy, but if you want to achieve consistent results, you need to be accurate!
The most accurate way to bake is to use weight measurements rather than volume, and that requires a good digital scale.
However, many recipes show only volume mesurements, and until you take the time to convert it, you will need to use the volume measures. You want nice heavy metal cups for dry measuring... thin ones dent too easily, then don't measure accurately anymore. My favourite set is made up of 6 cups rather than the traditional 4... 1/4, 1/3, 1/2, 2/3, 3/4, and 1 cup. Using the 2/3 cup measure once is going to be more accurate than using the 1/3 twice!
It's the same deal for measuring spoons... a thick heavy metal bowl will stay consistent for the life of the spoons. I was recently given a set of little spoons to compliment my primary set... it measures a dash, a pinch, a smidgen, and a nip! If you measure spices and things out of bottles, you may wish to get the narrow style which are designed for even the tablespoon to fit into a spice jar! (BTW... avoid plastic measuring devices if possible... they are often inaccurate and they stain and hold aromas and grease!)
Now for liquid measures (and please... don't use your dry and liquid measures interchangeable... you sacrifice accuracy every time!) I generally prefer glass measures, in the smallest size appropriate (i.e if I'm measuring 1-1/2 cups, I use the 2 cup measure, rather than the 4 cup.) I have recently found a little measuring cup from Oxo that measures in tablespoons up to 1/4 cup which is also very handy! It also has an angled scale which means you can read it from the side or the top!