Very Computer

I've recently been developing some Unix applications that I believe have
a marketable value, but I only have access to an AIX machine (and
Linux).  I was wondering if there are any services on the Net that
provide access to other Unix machines for software development?  It
would seem to me that Unix vendors would have machines set up on the
net for this purpose, but I haven't found any yet.  Also, which
platforms should I compile my software for?  I would like to cover at
least 75-80% of the market.  Would that only comprise about a half
dozen machines or so?