LeoCAD is fully free & open-source crossplatformed standalone software that could run on older PC too. Now availiable for Linux/Mac/iOS/Windows; port for Android also in plans:
Mecabricks is cloud-based online, so it require Internet connection and, I think, modern PC with WebGL support. License is EULA-like(?) and its track user (users are kids). Not good for security reasons, and not availiable in offline.
Stud.io is only for Mac/Windows, and its licensed as EULA-like(?). Not good for Linux users.
Here is comparsion of LeoCAD with some other LEGO CAD's (November 2015)
Stud.io and LDD both run well under Wine. However it's good to have an alternative tool that is open source. Stud.io uses the LDraw part database, the same as LeoCAD.
There's also blueprint, a tool to design LEGO instructions. It's not open source due to the author's legal concerns about publishing an open source library to read LEGO's own part database. It's a pity because it is a great tool, with a very good UI. LDraw files (.ldr) support instructions, but the user interface is a bit harder to use compared to the more specialized one in blueprint.
I am involved in the Mecarbricks development concerning the rendering features. I started a research in 2014 how to render LDRAW and LDD data in a 3d software. At present time it was not easy to get good working and looking 3D models out from LDRAW.
After the first success I started to import bigger models and got to the limits fast: complexity.
The Millennium Falcon with over 5.192 bricks took 16 hours to import to Blender and exporting to Modo. Finally the scene was 2.5GB and hard to deal with.
The solution for that is instancing but the Blender importer did not support this. Also the actual version is not working properly.
End of 2014 I found Mecabricks, contacted the developer and I shared my research knowledge with him and gave a list of features. While I was still working with Modo Nicolas Jarraud was developing a Blender Template based on my Modo setup.
Actually Mecabricks is very powerful because the editor is great, the database is constantly growing due to the help of users. Actually MB has around 10K users growing. Nicolas is working also on a Instruction Generator. The tool supports also import of LDD data depending on the database match. Compared to LDD MB is still not complete. MB supports textures and Blender. You can plan and render your MOCs in photoreal quality out of the box. With Blender it opens a lot of possibilities for animation and game design.
It's just online only. The community is a mix of kids, teenager and AFOLs. Also the designer saabfan of the Saturn V rocket is part of the community.
Here's a great example for the power of MB. Gabriele Zannotti is a young graphic designer who started with Blender and 3D in 2014 inspired by my first renderings. He's a great talent and used MB to promote this LEGO Ideas project:
Your comment look like promo material of commercial proprietary product in thread about free & open-source software... Its look like ads...
Anyway, what you use for import LDraw models in Blender?
Currently there is free & open-source add-on "ImportLDraw"[1] for Blender 2.7x that work well, but need few volunteers for little improve it. Could you help with it?
And the new tool from Stud.io from Bricklink? https://studio.bricklink.com/v2/build/studio.page