I work freelance so the only guaranteed common tools I use are a pad and a pen. Seriously. OK I have some sort of PC (usually Windows) and word processor. And email. Maybe.

I tend to extend this to my personal work philosophy in that I use the latest version of the Windows client, Windows VMs, Visual Studio, SQL Server, Office and IE. There are alternatives, however, these are the most common tools for each of the categories that I come across. I have heard other freelancers say things like "I must have Firefox" but I have worked at too many places where there is no choice. Certainly not for temporary staff/contractors/suppliers etc.