Arachni Web Application Security Scanner [1], is a Free/Open Source project, the code is released under the GNU General Public License, version 2 and you are free to use it as you see fit. On the January the 12th of 2012 the software Arachni is moving away from GPLv2 and towards Apache License v2.0. This is due to several license reasons [2].
[1] http://arachni-scanner.com/overview
[2] http://trainofthought.segfault.gr/2012/01/12/arachni-is-moving-away-from-gplv2-and-towards-apache-license-v2-0/
[1] http://arachni-scanner.com/overview
[2] http://trainofthought.segfault.gr/2012/01/12/arachni-is-moving-away-from-gplv2-and-towards-apache-license-v2-0/