Some apps can really change for good the user's experience, adding or replacing features. With this thread, I want a place where users can share the apps in their opinion are a "must-have", and maybe finding out new great apps they might not know about.
Please, do not post social media or game apps, because those are either known or entertaining apps, the idea is instead to share productivity and utility apps.
So, here's my picks:
SwiftKey Keyboard (free): This very useful keyboard really makes typing easier and quicker on mobile devices. It includes gigantic libraries for an huge amount of languages that is always growing, plus it learns fromusual expressions of that particular language, and it's own user's usual words and expressions - so instead of having it autocorrecting what you type for other words that make no sense on your phrase, it'll be way more accurate and it'll even predict those stuff you use to say regularly without even noticing. Not only it's autocorrecting and predicting system is accurate, it also has plenty of customization options, such as the size and placement on the screen and a lot of free and paid themes to choose from, allowing the user to customize it to the best design and functionality preferences of the user.
Google Now Launcher (free): If you have a Nexus device, then skip this one. If not, then you might be interested. One of the things I like about Android is the possibility to change the launcher (main menu) to one of the plenty to choose from, each featuring it's particular features. Google Now is, from those I already tried, the best imo, offering a simple yet practical design and some features you'd otherwise need to install multiple apps to have, such as news (that learn from your search to know your preferences, thus showing more content that you might be interested on; and on the other hand allowing you to choose themes and websites you are not interested to not receive any news from them) and weather from your current and regular localizations. It also offers a voice search system, if that's your thing (not mine though).
Opera Mini Browser (free): I first discovered this app when I needed a quick and light browser back to my first smartphone which had very low specs and couldn't run properly other browsers. Back then it was pretty much straightfoward: you had a light browser, offering you a "regular" mode that worked like other browsers, and a "high saving mode" that saved you internet, great if you use mobile data. Nowadays it has the same features and characteristics, plus many more - such as it's own built-in ad blocker, a "night mode" that reduces the brightness and can optionally give the screen a yellowish tone to be easier to read on low light without forcing the eyes and an option to receive Facebook notifications trough it.