This is something that I started this year in class and my students LOVE this. It is a very simple concept: I just write one note to a student each week or so - sometimes I do more, sometimes less and let them know that I noticed them doing something above and beyond what they were asked to do. It could be like the note above - which I just put in the mail yesterday - thanking one of my students for helping to get a friend's paper up to passing by taking time from his work to edit their paper. I have written them to students who went above and beyond on an assignment, who have carried books for their peers with injuries, who have given me a gift for a holiday, who have been really encouraging of a peer, who have edited essays for peers out of the kindness of their hearts, you get the idea. The point is: my students seriously LOVE to get these letters. I make the cards myself (but I do think this is probably lost on them) and I ALWAYS mail them.
Most of the students who receive these notes thank me for them the next day at school and I have even received emails from parents thanking me for taking the time to do this one little gesture.
Of all the little things I do to try to let my students know that I care, this is BY FAR the thing that they seem to appreciate the most.
This is also a secret classroom management tool: it is a way to let them know that I am paying attention, not only to their skills in English, but also to the effort they show and their compassion, etc. - things that can't be communicated through a simple grade or even two remarks on a report card.
It is strange, because this really is something that I do for my students, but I enjoy doing this as well and it really does help with classroom management and building rapport with students.