Geometric Calculator - Links
Though there's more than I found the last time I searched, the resources are still pretty thin in number and deep in content.
The Geometric Calculus R&D Page at Arizona State University features David Hestenes' research papers.
The Geometric Algebra Research Group pages at Cambridge University contains a variety of research.
Pertii Lounesto's home page at the Helsinki University of Technology in Finland contains links to many other Clifford Algebra sites.
HP Museum offers an explanation of and tutorial on reverse polish notation (RPN) calculations.
The International Clifford Algebra Society has links to books and abstracts of publications.