Ecosystem lock in is a rampant problem in the mobile industry, with every manufacturer preferring its own platform at the expense of others. Every so often there are exceptions, though. Like this collection of apps Microsoft made for Android, of all platforms.
Microsoft Office Mobile
It may be a bit of a gimme, but Microsoft Office Mobile shouldn't be overlooked in the Android space. The app allows you to view or make minor edits to Word, Excel, and Powerpoint files. It's not a replacement for office, but it's a handy piece of software to keep around.
OneNote
Another unsurprising, yet often overlooked entry is OneNote. Google's own note-taking app Keep is intentionally limited in its scope of features, while Evernote pours on the love for power users. OneNote strikes a nice middle-ground sweet spot. You can organize pages into notebooks like you can with Evernote, and you can scribble quick notes with text, pictures, audio, and even drawings like you can with Keep. It may not be for everyone, but if you're not married to any note taking app yet, it's worth a look.
Next Lock Screen
Credit where it's due, Microsoft made a pretty neat lock screen. Carrying the inventive name of Next Lock Screen, this app shows you an agenda-style list of your notifications that you can swipe to dismiss or tap to go to the relevant app. It also includes a list of commonly-used shortcuts along the bottom. Drag this drawer up and you'll find a few more apps, as well as system shortcuts like the flashlight or Wi-Fi and Bluetooth toggles.
Smart Glass
For Xbox users (both 360 and One), the Smart Glass app is indispensable. Not only does it allow you to access companion information while watching shows or playing games (such as seeing maps of Westeros during Game of Thrones), it also allows you to control your console's entire interface. You can access apps, scroll through messages, and navigate menus with the touchscreen on your phone or tablet. In many cases, this is much nicer than using a controller. Like, for example, when entering your Wi-Fi password.
Xim
Xim is a weird little app that's actually a bit more useful than it seems at first glance. It allows you to create a collection of photos (called "Xims", because everything needs a silly name) and share it with someone. The trick is that, while you and your recipient are both viewing a Xim, you can swipe between pictures and it syncs between devices. You can also message each other within the app. Xims don't share photos directly and the recipient doesn't need to install the app to share the experience, making it a useful way to share photos in a semi-disposable way. It doesn't promise a ton of security (users can still take screenshots), but it's handy.