SQLite is an automatic, and easy, answer. It almost goes without saying at this point, I guess.
vim is everywhere. I like knowing I can always run it no matter where I am or what server/VM I'm logged into. That knowledge provides comfort.
go. I don't actually love the language, but the solidity of it and it's ecosystem is really nice. Everything around go has a patina of workmanship about it; not in a corporate "professional" sense, but more like a well-made wood plane or something.
Perl. Again, I don't entirely love the language, but the incredible backward compatibility (we have ~20 year old code that still runs unmodified!), the rarity of bugs (especially given how big the surface area of a huge language like Perl is), and the overall culture of testing, leads to a pretty nice experience. I feel like I can count on it, even if I might get grumpy at some of the quirks.
Python, for different reasons from Perl. I can come back to Python after years away and find that it's maybe even a simpler (but not less powerful) Python than I used a decade ago. I just poked at Python 3 for the first time a few weeks ago, and found it immediately readable and intuitive. The language is more reliably "Pythonic" every year.
OpenSSH. Always works. Always trustworthy.
Google Inbox. I want to host my own mail (and do, for my business), but nothing is as good as Inbox.