Good online reference for Euclidean geometry.

I was trying to help someone understand a script on the preview grid last night, and it eventually dawned on me that he didn't have even a basic understanding of vectors and vector arithmetic. I'm not talking about quaternions or matrices, he was having a rough time understanding what a unit vector was.

Is there a good online reference I can point people at for this kind of thing?

Try this link:

http://www.euclideanspace.com/maths/algebra/vectors/index.htm

Thank you. I've also been searching such an information by chance.