It goes without saying that R would not exist without the pioneering work of John Chambers and his AT&T collaborators. John has changed the way that many of us think about statistical computing and the fact that R has evolved to resemble S as closely as it does is is a testimony to the extent that people enjoy doing data analysis with S.
The Free Software Movement (or movements perhaps) has been another major source or ideas and influence on R. I do my development work on a workstation which runs the FreeBSD operating system and is equipped with a rich set of development tools from the Free Software Foundation. Hopefully, R represents some pay back to the free software community for what they have provided to me and the other R developers.
Finally, I'd like to acknowledge my partner-in-crime; Robert Gentleman. During our work on R we have practically lived in one another's back pockets. It speaks volumes that, not only are we still on speaking terms, but we still go out for beer together on Friday evenings.