|Look Ma, three racks!|
These are the 5 tools that I use all the time and wouldn't want to give up, in no particular order. (I'm not including things like spatulas and measuring cups, those are too obvious.)
1. The Agbay cake leveller. Once you use this you'll wonder why it took you so long to buy one.
2. A good turntable. A good turntable can make icing a cake smooth 100% easier. it doesn't have to be expensive, it just has to turn smoothly. I use a wooden lazy susan and it works great even for really heavy cakes.
3. An extra oven rack. I hear a lot of shrieking from people worried that they can't bake more than one cake layer at a time because the oven won't heat it properly. I'm here to tell you that if you know how your oven heats, you can bake off an entire three-tiered cake at the same time. I bought an extra rack for my oven and it cut my baking time down substantially. You just have to know when to rotate pans, where the hot spots in your oven are, etc.
4. A five-petal rose cutter. Good for quick roses and other petally-type flowers. Get a few sizes for variety. PME Sugarcraft Flower Cutter Set - 5-Petal - Large
5. A silicone pearl mold. I use this all the time and wouldn't want to go without it. It makes sizing fondant pearls super-easy. Even though I re-roll them and don't use them straight out of the mold, it cuts down on my pearl-making time.
What are some things that you use to decorate cakes that you wouldn't want to give up?