What is your favourite Emacs hack?

Do you mean clever thing to do with it, or literal hacks?

For the later the the movemail exploit, as documented in Clifford Stoll's Cuckoo's Egg book was pretty good.

I reported a couple of trivial predictable-filename issues with bundled lisp too - including one that was related to Mosaic which dates pretty badly:

https://bugs.debian.org/747100

I guess "neat elisp" is such a wide topic people can and do write lots on the topic. I'll just say "org-mode is awesome".

Just in general, Emacs first class treatment of the "old school" IDE features. Wiring up with grep output and ctags files. It's nice to be able to jump right into any project, any language, with minimal setup and get cracking.

M-x compile is nice.

C-h C-t