As a writer, I'm very much interested in free writing programs. Ever since high school when my Microsoft Office self-destructed on me, I wasn't about to pay $$ to get it again. So I embarked on a quest to discover alternative programs.
For general writing/Office
1. First to top this list is Google Drive. I LOVE Google Docs. I use it all the time. For me, the only drawback is that if you lose internet connection, you might not be able to use your document. But there are ways to back up and save what you wrote. One of the greatest features of Google Drive is that it's extremely easy to share with other people for review, feedback, and collaboration. It's real time collaboration is a little freaky and distracting at first, but I think its amazing. I also love its inline commenting system and the fact that you can have chats with someone else while working on it. You can add pictures, all sorts of fonts, and so many other things. Plus the other parts of Google Drive is awesome too, including Slides, Drawings, and Sheets.
2. If you want something that isn't online, your best alternative here would be Libre Office. I use Libre Office when I'm opening up Word documents on my laptop and it's essentially the same thing, only free, and it has slightly less features. But like Microsoft Office, it also has graphs/charts, excel sheets, drawing, documents, and presentation.
3. If you're still stuck to Microsoft, you can still use their online version, Office, which doesn't have all the bells and whistles, but is still the familiar program you love with a couple little extra things like calendar.
4. I was completely devastated to when I lost Microsoft Note. I LOVED the ability to organize things and take random bits and pieces here and there. Thankfully, I have Scrivener now which I use for my novels, but back in the day, I was using Microsoft Note to keep a running "guide" to my novels with everything about it in them. Evernote though is a more web-friendly, device friendly, version of Microsoft Office that I actually like better. If I wasn't using Scrivener though for my novels, I'd be using this to keep track of my info. But for the organized person, I definitely recommend Evernote. Plus, it's free.
5. "Notepad", if you have a PC with Windows, I honestly say that if you want something simple and basic as it gets, I would say just use your built in Notepad or whatever the Apple version is (I think Apple gives you Pages?). I use notepad for keeping notes and writing down random things. It's also great for getting rid of weird formatting if you're copying and pasting text from other applications.
6. Although it's more oriented to businesses, I've used Zoho before myself and it's pretty good. It has a ton of applications within it's online platform and it's free. So if you're writing for a more business oriented view, I'd say using ZoHo wouldn't hurt.
For Longer Documents/Fiction Writing
1. My #1 recommendation is Scrivener. It isn't free, it costs $40, but it is AMAZING. ABSOLUTELY AMAZING. Even if you don't write fiction, it's amazing for long papers, or anything that you need kept organized. It has a TON of features, most that I don't even use because it just has so much. But I fully support this amazing software. You can use it on both Macs and PCs and it has a $14 IOS app.
2. This free alternative to Scrivener isn't as great, but it's up there. yWriter is pretty good, with similar features, but doesn't have nearly as many. Also, the website is a little outdated, but I've heard good things about it.
3. The other recommendation I have for this category is PageFour, which layout wise is similar to Scrivener, but also doesn't have as many awesome things as Scrivener does. It's also free, so feel free to try it out and compare it to yWiter.
4. Created by the Writer's Knowledge Base, WKB (what I like to describe as a "google" for writers), Hiveword is a free, online program that allows you to better organize your novel.