So many things. Mentioned a couple already: VSCode and Hyper. A couple recent additions that I really love: the VSCode prettier extension that gives me format-on-save, and the Git Lens VSCode extension which shows inline git blame annotations. I use blame ALL the time for ECMA262 so having it inline has been a huge time saver.
Another task I do very frequently is compare how snippets of code run across various runtimes (Chakra, v8, Spidermonkey, JSC, etc). I build a tool called eshost that abstracts the differences between these runtimes and compares the results of them. This has probably saved me days of manual effort.