The single most useful Emacs feature
hi-lock-mode!<p>Hi-lock lets you specify regexes to highlight anywhere in a file; it's like domain-specific font-lock.<p>It's <i>astoundingly</i> useful in code review. For instance, given a giant blob of J2EE web handler code, I can eyeball one handler, recognize the annotation mapping the method to URL syntax, and then punch in a hi-lock regex to light up similar annotations everywhere in the file, then glance through the whole file looking for color patterns. I can read any of those functions and recognize the code that checks "am I an admin", hi-lock it, and then quickly spot any function mapped to a URL that doesn't check admin credentials.<p>That's just one tiny use case. I used hi-lock so much I ported it into our web-based code review tool. I converted one person to Emacs just to get that feature (my office would be thrilled if someone could point out the vim package that does the same thing).<p>CUA column editing (CTR-SPC arrow-arrow-&c type-type-&c) is probably my #2.<p><i>Late edit:</i><p>Holy hell how do you forget magit! Magit is a near-complete UI for git; it figures out what git repo you're working from, and then gives you up-to-date status on the contents of your checkout, spots untracked files, allows you to visually stage your commits, gives you single-key revert and a browsable repo history.<p>Magit is one of the rare Emacs extensions that not only improves Emacs but also the external service it integrates with. I am actually a better git user (or, git is just better) when I've got magit running.