User blog comment:Renzari/How?/@comment-2156149-20130712191430

The only software I know is LEGO Digital Designer (version 4.3) which can be download for free on Lego.com.

Any other softwares that the "pro" builders use to make their Lego MOCs or Bionicle MOCs are presumly the ones that you have buy it and some of them probably requires the user to have some experience in computer graphic or graphic design (or something like that).