Letting another author know I'm using one of their results Let's say I write a paper and use a result from someone that is rarely cited. Would it be polite/acceptable/unacceptable to email them and let them know of my application of their result before publishing my paper? 

This would be primarily self-serving on my part, as I'd want to know if they see any other applications of their work to mine.  But I don't want to come across as someone who's trying to leach ideas off of them. I'm open to collaboration.

What would be your reaction if you received an email saying something to the effect of:


  Hey, I used result X from your paper from 10 years ago to show Y.  Do you think other applications of your related work could apply to mine?  Here are some of my questions, etc...
Citing another is in no way "leaching". It is the natural course of scholarship. It took a long time for my dissertation to be extended, but I was very happy when it occurred. I was, in a small way, an inspiration for another. That is what we do in academia. 

Send the email. Moreover, wait and hope for a similar email to come to you in the future. Too seldom do we get to actually thank the people on whose work we build. 

And if the person is still working in that field, asking for collaboration is completely proper. In my own example, I'd left the field and had nothing to contribute going forward, but it was still a nice feeling that someone, somewhere, still cared.