I could have sworn there was one that I encountered recently, but I can't remember the name of it. There was a really old one called WRUGS, but it hasn't worked for ages. I did some Googling and found someone mentioning they updated/fixed/changed their own copy of WRUGS for themselves, and released it as an addon called BlockU. I have no idea if it works or how well, but here's a link to it:
http://wow.curse.com/downloads/wow-a...ls/blocku.aspx