Ideas for DIY solid surface kitchen counter top?

I am not sure what your budget is, but marine epoxy (I have used West System) can be pretty tough. You can put a thick coat on any number of materials, I have seen it over a coat of pennies, and I have experimented with it on OSB.
Solid wood counter tops are another option, if that's a style you like. Laminate boards together with epoxy and you have options where to get your material. Consider doing it end-grain up; it won't seal so well but wood is apparently toughest in that orientation (butcher blocks are like that, and so used to be flooring in mechanics shops in Europe for the longest time).
We have actually had some plywood, solid wood edged and then painted countertops for several years now. I initially put them down because, like you, I was looking for something "nice" and needed some horizontal surfaces in the meantime. I am amazed how well that has lasted, and almost wish I had simply gone for a harder plywood.
A last option... check out your local plastics supply place and see if they have any material that suits you. Or flooring... think battleship linoleum, almost unkillable.

I have been thinking about this too and I just found a really cool old trunk on craigslist for $50. This piece is beautiful old oak and has already been perfectly cut for a piece of furniture, so I don't need to make too many cuts. I am going to put an underlayment of plywood for the entire countertop, then I am going to take the two long front and back pieces and use those for the bulk of the counter top (the trunk is 50" x 20" x 18", so these pieces will make a 50" x 36" piece together. At the end I am going to use the two end pieces (which are already cut to perfectly match up with the ends of the pieces I am using). These are trimmed now and I may leave them that way as a decorative touch, although it does make the counter less smooth. There are trim pieces on top of the large pieces of wood I will use to cover the edges. I am sanding and finishing this with Rubio monocoat, which just requires one coat and is food safe. I should have some extra pieces that I can use to add a bookshelf at the end of the island.

I have a beautiful old house with great wood beams and floors, so I think this old oak is going to be freakin' gorgeous. I just hope I can pull it off. Will make and instructible if I turns out well.

Another completely different option is to go for stainless steel counters. We recently did this in our kitchen and if you're willing to do some of the work yourself then you can really get it done very cheaply - our small counter ended up costing around $400.

How-to steps: DIY Installing Stainless Steel Counters

Anyway, it's durable, looks great and is very affordable!

I knew a bar in Newcastle, where the 'top' was clear resin over compact discs and other stuff. If you get a wood or particle-board base you could pour a resin finish on it. It might need a final sand and polish, but you'd have flexibility in design and a relatively lightweight item. L

Working in the industry has shown me a lot of different ideas and projects people have come up with but the simplicity rule sometimes makes for a better project depending on your skill level, A elegant countertop is not limited by budget but creativity. a simple idea using shop grade plywood and hard wood edging to surround tiles that have been grouted, then sealed with a clear epoxy can be very luxurious while still allowing you to customize your design and color scheme

Concrete tops are not all industrial looking, I've seen where they have pieces of glass in them, colorants, acid washing, fossil's,glazes, inlays and stamps, etc they look awesome, yes most are cast and then diamond polished to achieve a glass like finish... And yes it is heavy, but if you do it right with steel mesh and use the right additives it is strong, very strong and normally doesn't crack unless abused, but if you abuse any counter it will bust or crack... On a concrete counter top you can mold a sink into them complete with a dish / veggie draining area, I have even seen metal strips cast in the concrete to allow a cutting board to be attached to it magnetically, and a lip around the sink so you can put a nice board over top of it and still have a level counter top, even a built in towel hanger... And if your feeling real eager you can also cast a back splash on it....

I just saw a guy resurface all of the laminate countertops in a kitchen with concrete... for under $200. There are a lot of steps to the process, but it looks like something that you can do without carpentry and building a lot of forms.

http://www.hgtv.com/hgtv/dc_design_kitchen/article/0,1793,HGTV_3375_5455954_03,00.html

You can also hire a contractor to do this for you. I found a few in my area that offered this service.
:-)

I saw a really cool concrete countertop
http://www.instructables.com/id/Concrete-Countertops-for-the-Kitchen---Solid-Surfa/

Do you think you will still need intense pressure and heat to cast a slab of composite material? Even a quart of casting resin is pretty expensive for my taste so doing this on a larger scale might still be prohibitive. You would have to find a heat resistant and machineable hard resin. I don't think you can even buy scraps of commercial composite countertop because the manufacturers certify the installers and keep the market to themselves. I just went with tile on my countertop. Next one I hope to use granite tile with fine joints. That is the closest I'll ever get to a luxury countertop.