I think a good baking stone is absolutely essential to making good bread, great pizza and flatbreads. The most important thing when looking for a stone is the thickness, without a good solid amount of weight behind a stone the bread you're cooking will quickly cool down the stone, and you may as well be baking on a sheet tray. A thick stone can set you back some serious dough, but really a thin cheap stone just isn't going to work well, it doesn't have the thermal capacity to keep your dough toasting on the bottom.
This model is a great thickness at just over 1/2 inch, it's not so heavy that it'll take forever to heat up, but not so thin that it doesn't hold enough heat to properly brown your crust.
One thing to remember, never let your stone get wet, if it gets wet the water will seep into the stone and the next time you put it in the oven it'll crack or shatter. If you do get it wet, make sure you dry it very very well and put it in a cold oven and very slowly bring up the heat to release the water slowly.
Another item essential for baking is a scale, because flour will compress so much two people getting a cup of flour can be as much as 50% difference in weight! When you're working on perfecting your bread recipies it's absolutely essential that you weight things out. Also if you look at the serious bread forums like thefreshloaf.com almost all of their recipes will be by weight.
I like this model of scale because it's small and easily stow-able. It has a tempered glass weighting surface for easy cleaning as well. It's not the cheapest model out there but you get what you pay for with scales. If it's cheaper it will be less accurate out of the box and will quickly become useless.
Lame (pronounced lahm)
This is a curved Lame for scoring the perfect little ears of bread on your freshly baked loaves. It's not as expensive as many of the other models out there, but it'll get the job done. I'm including the prettiest lame I've ever seen as well just for fun.
The second one is also handcrafted in the USA if that is important to you! -Nom!