You might be thinking of the Dev network keys (MSDN).
MSDN used to be free so this was a common approach, but they re-org'ed all their programs so I think those keys are now hiding under the paid MSDN program...
If you have a university that supports alumni, Microsoft grants a key to each student, faculty, and alumni in most of their post-secondary education plans.
My other recommended way is through the Startup Hub.