Currently, our focus regarding software development lies on Internet applications, which we write in Python, although we can also develop using older programming languages like C, Perl, and a number of other languages, if need be.
With web applications at least, we favour test driven development. Our version control system of choice is Git, our favourite CI server is Jenkins, and we track our projects using Redmine.
If we collaborate on software projects with external entitites, we also use a variety of other systems that might already be there, like eg. Github, Trac, Bugzilla, Jira, etc.pp.