acrylic box with some other handy bits, but when I'm on the go I use a Ziploc bag. Kinda embarrassing.. I should probably get my act together and make a bag or something.
Anyway - this is my kit:
A pincushion - because apparently leaving pins and needles in the arm rest of the sofa is not an acceptable way of storing them. ;-)
An assortment of hoops in lots of sizes, from 3" to 12". Who knew that you'd need atleast one of pretty much every size? And of some sizes I have multiple, because changing hoops between projects is just too much like hard work...
Regular sewing thread is really handy for temporarily stitching excess sections of fabric around the hoop so the excess isn't in the way of working on the project. Also for holding things in place when doing appliqué.
I use this tracing pen by Hemline for most things. It's just so convenient to use and it rinses out without a trace most of the time. And it's fairly affordabe, which is nice too.
Sometimes I also use Saral paper (not pictured) for transferring patterns, usually on darker fabrics.
That wee blue cube is a great little embroidery 'gadget'. It is a floss conditioner which is a waxy-ish substance that you run your thread through and it helps it stay smooth. I don't use it that often, but if I'm working with satin or specialty floss, which splits easily, this thing is gold!
Also in the gold category are my Fiskars embroidery scissors. I love them so much.
I have a seam ripper in my kit as well because sometimes mistakes can't be saved or ignored and out they must be ripped.
Needles, of course. I started out buying those packs with different sizes in, but I soon realised that I always go for the same size, so now I just buy packs of that. It's handy to have more than one of the same size needle in case it goes missing. In the sofa or whatever. Not that I have ever tried that of course. ;-)