Specifically CLion's excellent code browsing features. It's the only thing between me and insanity when I have to "quickly" patch a binary with source code spread out over 1000+ files.
Especially local history - being able to go back in time before any commits were made saves you rewriting code or having to remember what something was doing before.
Otherwise the number of times I "show usages" or refactor shortcuts (rename, extract method / variable) throughout the day...
All of it out of the box as well, it's great value for money.
