The one material that I found was much more customizable than the other solid surfaces was concrete. The colors and textures are endless, you can mold it into just about any shape you can imagine, and it retains the advantages of solid surfaces. But it's concrete, so it must be cheaper than milled stone, right? If you have a professional do it for you, it can actually cost MORE than other solid surfaces...do it yourself, and you can save a bundle. I priced granite countertops in my kitchen and they came out at around $4,000. My custom concrete counter tops ended up coming in at below $800, including the rental of the concrete mixer.
Now before you get ramped up and ready to pour, I will preface all of this with the drawbacks to doing your own concrete countertops.
+ This is not a project that can be completed in a weekend. No matter how small the countertop is, there's at least a 10 day curing process which you will need to do your grinding and polishing in.
+ Concrete needs to be properly sealed at the beginning and waxed about every 30 days to avoid staining.
+ The final outcome might not be exactly what you expected, especially if you're doing it for the first time. The good news is that there's ways to remedy many outcomes that you may not like.
I highly recommend that you buy the book Concrete Countertops by Fu-Tung Cheng before attempting ANY concrete countertop project. Fu-Tung Cheng is the Master and if you're looking to be the Karate Kid of concrete countertops, buy this book. Ralph Macchio wouldn't even think of doing concrete countertops based on my instructable.