http://www.indiegogo.com/Terrablox
This is a project I've been working on for a few weeks and trying to make a reality.
The idea is explained more in depth on the page, but it should be pretty self explanatory. These are just concept renders, I'm prototyping several different designs, with a priority on making sure they stay together firmly in all directions, so that you can build things such as the tree. I found a local 3D printing place that's making the prototypes for me and as soon as they're done I'll upload video of what they look like and how they work.
I submitted it to Kickstarter, but they rejected it; it wasn't really fleshed out enough and I had no way to show people would be excited about the project. So right now I'm just trying to generate as much discussion as I can, so that Kickstarter is more likely to accept the project. However, I do have donation goal; that's not to actually produce the things but just to put towards the upfront setup costs of setting up the factory. The more donations I get the cheaper the final product will be and the fewer pre-orders I'll need.
The page is not to try and sell pre-orders, so if the idea of charity turns you off just wait until it's on Kickstarter, and please help in other ways like showing as many people as you can :D
Let me know what you think; the more buzz I get the more likely it will become a real product.
That's a cool idea, but you can already buy these, as described, at teacher supply stores. If you're bringing a little more to the table, you should punch up the description a little.
I looked before I put this up, and I could only find copies of LEGO, plain blocks, and cubes that only stack vertically except for one brand of 2cm cubes that has round holes in each side and links in every direction; neither the size nor holes make it ideal for tabletop RPGs.