C-M-u-q is one I use a lot: it auto-indents code (useful after you've made changes to a function or macro definition).
Emacs help actually defines these as two separate commands (C-M-u is "backward-up-list", C-M-q is "indent-sexp"), but I find that I can just hold down C-M, and then type "u", "q" in succession for the same effect.
Actually, I don't use C-M-\ much, since I prefer to work in individual blocks and C-M-u-q does that nicely, without my worrying where I last set the mark.