Back in 2004 I did a small plugin for Miranda IM messenger which was providing auto replying facility. I uploaded it to Miranda database and since that time it had ~ 40 000 of downloads. Some people were inspired with it and based on it some other plugins: Simple Auto Replier
Recently I came across this code and decided to make it GPL and put in Google Code project. Here it is: http://code.google.com/p/smart-auto-replier/
It was fun to look at my own code which is 8 years old.