I am a professor, and - let me tell you, a formal letter to my Dean is a huge thing for me, especially in my case, because I go up for contract renewal every three years (i.e. I am not on the tenure stream).
And - even if your professor is on the tenure stream and/or even if he or she has tenure, a letter like this will still be helpful - especially if he or she is going up for promotion soon (assistant professor to associate professor, associate professor to full professor), as all professors have to demonstrate their teaching and service contributions to their school.
So - write the letter (addressed to the Dean/Department Chair) and be very professional. If you sound too gushing or if you go into extreme detail, the letter will lose its effect. Just be sure to spell out what your professor did and why it was above and beyond the call of duty. Use your full name and permanent address (so that you could be contacted as a reference in the future), print it out on nice paper, sign it, and send a copy to the Dean and/or Department Chair. Then, put a copy in an envelope, with a card with a short handwritten note, and give it to your prof. I guarantee you that he will be flattered/honored that you made such an effort to recognize him.