I present you, QuickDiff: diff.mattiasgeniar.be.
It does a very simple job: it takes the input from 2 text fields, and visually shows you the differences between them. It's a graphical layer on top of the "diff" command in Linux.
There are already sites out there that have similar (and more advanced) functionality like this, but since I'm paranoid I want to build it myself. I give you my word that I don't save your input, but I can not prove it in any way. And that's the exact reason I don't want to use someone else's tool: when I use it to diff config files, I don't want that to be sent elsewhere.
Up to you if you decide to use it!