As a long time nano user, who found his way here looking for something equally simple with a real undo/redo buffer and standard GUI editor key bindings, I heartily recommend the Sanos editor. It is tiny and does just what the original query demands. Its sole defect is there is no mouse support for placing the cursor. It is my new favorite minimal editor, and nano is going into the dustbin. You do have to compile it, but it is a trivial program to compile - a single C language source file. I compiled it to "sane" for "sanos editor" (sanos is actually a mini-os).