Clifford Algebra - Geometric Calculus
Tim Sweeney sent along the following bit which you may find useful and/or interesting:
I've been reading up on Clifford Algebra (heard about it from a link on your
site), and it's REALLY cool. While I haven't done significant programming
with it yet, the symbology of common vector and matrix algebra operations
with Clifford Algebra is a lot nicer than with matrices. I think this is
definitely valuable to hardcore 3D programmers.
Two really good books I've found on the topic are: