As has been said by others - very helpful and provocative thread. Had a potential web project with a local professor. Didn't come off but he was very apologetic that it didn't come off and complimentary about my services. Your article made me think - why not jot him a line and ask for a link from his site anyway! He put the link on within minutes of me emailing him.
Don't want to hijack it with my questions but - about your calculators. Do you put them onto your site and ask people to link to them or on their site with a link back to you? I have something similar-ish - One of my sites charges for people to do practice tests using official government licenced questions in a flash quiz format. I could offer a free version with limited number of questions.
Using your calculator experience would you think it best to:
put it on my site and ask others to link to it (likely to get low return)
offer it to others to put on their site and ask them to put a link to me on their site
offer it to others with a button at the end saying "now get the full version"
or any other way of using it to generate a link