Go for Microbrush-3-wip-2014-12-17.2.zip, just fixed a configuration mistake preventing the toolbar from working right. 
Sorry.
Hm, I only have capabilities to support the Windows platform at the moment. But because the editor uses bare Win32 API, OpenGL and .Net, it should be possible to run it with Mono and Wine.
I have just released a new work in progress version - the one used in the video - at http://shrinker.beyond-veils.de/projects/IterationX/Microbrush3/.
It should already be pretty decent to use for a bit of productive brush making.
Here is a little demo of me building some stuff live:
Making progress with the interface... an actual workflow is slowly forming.
This random stuff here took somewhere between 1 and 2 hours of build time.
[Blocked Image: http://shrinker.beyond-veils.de/temp14f/2014-11-28%20mb3%20random%20stuff.jpg]
New maintenance release: http://shrinker.beyond-veils.de/projects/IterationX/Microbrush3/
- when quitting with at least one view open, it now asks if you really want to
- fine-tuned the grid a bit
- can now reliably merge a big load of brushes (fixed the algorithm)
- brushes not affected by an operation are not mysteriously hidden anymore under some circumstances
- turned off the render data cache by default, then verified: It can handle more than one million brushes in a world. 
Yes, one million, because why not. 
New version with the rotation and clipping tools now implemented.
3-point clip:
[Blocked Image: http://shrinker.beyond-veils.de/temp14f/2014-11-15%20mb3%203point%20clip.png]
Now this was quite a bit of work...
[Blocked Image: http://shrinker.beyond-veils.de/temp14f/2014-11-12%20mb3%20clipping.png]
Clipping is coming as intended.
The two different patterns you see in the selection mask on the right side indicate two different selection sets that you can independently deselect or delete after such an operation.
Thanks!
Steppenwolf: The editor's API can save and load the editor's own proprietary binary format. In the tools folder, there is a conversion tool that can read and write this and the VMF format.
When saving via the API call, you can configure it so that the calculated polygons are saved into the file too so they are available for conversion.
So yes, later on it will make a lot of sense for me to add such widely accepted formats as a conversion target, and the infrastructure is there and working already. :3
nicoreda: Unfortunately, the big advantages of these graphical primitives that actually represent 3D volumes are in danger of being forgotten... a peek at the new Source 2 engine (DOTA 2) shows that they are now using meshes for everything, which is kinda sad. But I'll continue supporting brushes with this tool. :3
Castle: Haven't heard of TrenchBroom before, but that one looks interesting too.
There's a predecessor to this new editor: http://shrinker.beyond-veils.de/projects/Microbrush2/
And to that, there is a predecessor too: http://shrinker.beyond-veils.de/projects/Microbrush/
Campaignjunkie: The brushes are valid for compiling, converting an MB2 scene to .VMF and back has always been supported.
I bet he meant that previously, one could use the arrow keys to adjust the texturing, while they now nudge the camera around.
Well, thanks. And the good news is: I'm still working on it! But right now, university tasks are eating 230% of my time, and I am experimenting with random map generation once again
That was already possible before, but the values were rounded when they were written back into the text fields.
How about another tool as a temporary workaround to get things done? Microbrush
