Setting up a virtual company is easy.
You need tools to manage your , usually, small group of people from all over the world in a consisten way. Robin Good posted excellent list of these collaboration tools available online for free or for a small fee .
It's basicly everything you need to work on a project from your home and collaborate with other people in the virtual company.
The mix of these tools depends on the type of project you work on. So, here's my selection of these tools for simple online collaboration based on my experience, familiarity, practicality and value ( relative price ) :
E-mail, IM, web phone, chat:
Gmail ( Google account )
But the aim is to keep it simple, easy and uniform. To be clear, these are basic collab tools ( for internal process ) and do not include ''external'' web services you use for other business purposes like PR, marketing, sales or business productivity ( Facebook, Twitter, Google Analytics, AdSense, SugarCRM, Highrise, YouTube etc. )