Some C programs I've written, which I've gotten around to putting into PHP/Javascript

One for the discerning millimetre-wave astrophysicist studying the absorption of molecular gas (and who isn't these days?)...
Column density calculator for common rotational transitions

Kneecap protection with the ...       Loan, debt and mortgage calculator

Other Useful Programs which others have already got running on the web

Binomial Probability Calculator

Cosmological Calculator

... although I have my own versions in C if you need them